Instead of trying to convince setup.py to do what we want, the wheel
package empowers us to unpack and directly manipulate the wheel.
In particular this is important because `--skip-build` really does
not work.

First we build a native wheel. If skip_host is True, then this
"native" wheel has defective empty files as binaries, if it's False,
we build a real native wheel. If skip_host is False, then we use
pip to install our native wheel into host directory.

Then we unpack the wheel, replace the platform tag with
wasm32_emscripten, remove the non-metadata files, and copy in the
new files with the cross compiled binaries.

Of course, this is all only necessary for binary extensions.


Hopefully this will help get us towards using alternative build
backends. The one concern is that this approach depends on
implementation details of where the build backend puts the files
into the directory tree before copying them into the wheel. If
the build backend moved the linked binaries after building them,
we would be in trouble. Or worse if the build backend deleted the
files after packing them into the wheel, we'd be in trouble.

Basically, it seems clear that we can only support alternative
build backends on a case-by-case basis.

@henryiii I'm curious what your thoughts are cross compilation + arbitrary build backends.

Better package cross compilation. Instead of trying to convince setup.py to do what we want, the wheel
package empowers us to unpack and directly manipulate the wheel.
In particular this is important because `--skip-build` really does
not work.

First we build a native wheel. If skip_host is True, then this
"native" wheel has defective empty files as binaries, if it's False,
we build a real native wheel. If skip_host is False, then we use
pip to install our native wheel into host directory.

Then we unpack the wheel, replace the platform tag with
wasm32_emscripten, remove the non-metadata files, and copy in the
new files with the cross compiled binaries.

Of course, this is all only necessary for binary extensions.


Hopefully this will help get us towards using alternative build
backends. The one concern is that this approach depends on
implementation details of where the build backend puts the files
into the directory tree before copying them into the wheel. If
the build backend moved the linked binaries after building them,
we would be in trouble. Or worse if the build backend deleted the
files after packing them into the wheel, we'd be in trouble.

Basically, it seems clear that we can only support alternative
build backends on a case-by-case basis.

@henryiii I'm curious what your thoughts are cross compilation + arbitrary build backends.

#!/bin/sh
#
# An example hook script to verify what is about to be committed.
# Called by "git commit" with no arguments.  The hook should
# exit with non-zero status after issuing an appropriate message if
# it wants to stop the commit.
#
# To enable this hook, rename this file to "pre-commit".

if git rev-parse --verify HEAD >/dev/null 2>&1
then
	against=HEAD
else
	# Initial commit: diff against an empty tree object
	against=$(git hash-object -t tree /dev/null)
fi

# If you want to allow non-ASCII filenames set this variable to true.
allownonascii=$(git config --bool hooks.allownonascii)

# Redirect output to stderr.
exec 1>&2

# Cross platform projects tend to avoid non-ASCII filenames; prevent
# them from being added to the repository. We exploit the fact that the
# printable range starts at the space character and ends with tilde.
if [ "$allownonascii" != "true" ] &&
	# Note that the use of brackets around a tr range is ok here, (it's
	# even required, for portability to Solaris 10's /usr/bin/tr), since
	# the square bracket bytes happen to fall in the designated range.
	test $(git diff --cached --name-only --diff-filter=A -z $against |
	  LC_ALL=C tr -d '[ -~]\0' | wc -c) != 0
then
	cat <<\EOF
Error: Attempt to add a non-ASCII file name.

This can cause problems if you want to work with people on other platforms.

To be portable it is advisable to rename the file.

If you know what you are doing you can disable this check using:

  git config hooks.allownonascii true
EOF
	exit 1
fi

# If there are whitespace errors, print the offending file names and fail.
exec git diff-index --check --cached $against --

#!/bin/sh
#
# An example hook script to verify what is about to be committed.
# Called by "git commit" with no arguments.  The hook should
# exit with non-zero status after issuing an appropriate message if
# it wants to stop the commit.
#
# To enable this hook, rename this file to "pre-commit".

if git rev-parse --verify HEAD >/dev/null 2>&1
then
	against=HEAD
else
	# Initial commit: diff against an empty tree object
	against=$(git hash-object -t tree /dev/null)
fi

# If you want to allow non-ASCII filenames set this variable to true.
allownonascii=$(git config --bool hooks.allownonascii)

# Redirect output to stderr.
exec 1>&2

# Cross platform projects tend to avoid non-ASCII filenames; prevent
# them from being added to the repository. We exploit the fact that the
# printable range starts at the space character and ends with tilde.
if [ "$allownonascii" != "true" ] &&
	# Note that the use of brackets around a tr range is ok here, (it's
	# even required, for portability to Solaris 10's /usr/bin/tr), since
	# the square bracket bytes happen to fall in the designated range.
	test $(git diff --cached --name-only --diff-filter=A -z $against |
	  LC_ALL=C tr -d '[ -~]\0' | wc -c) != 0
then
	cat <<\EOF
Error: Attempt to add a non-ASCII file name.

This can cause problems if you want to work with people on other platforms.

To be portable it is advisable to rename the file.

If you know what you are doing you can disable this check using:

  git config hooks.allownonascii true
EOF
	exit 1
fi

# If there are whitespace errors, print the offending file names and fail.
exec git diff-index --check --cached $against --
