Q:

How can I make git tell curl to use openssl instead of gnutls without rebuilding the package?

We have an internal server that GnuTLS doesn't like, e.g:
gnutls-cli --x509cafile /etc/ssl/certs/ca-certificates.crt foo.example.com
Processed 173 CA certificate(s).
Resolving 'foo.example.com'...
Connecting to '1.2.3.4:443'...
*** Verifying server certificate failed...
*** Fatal error: Error in the certificate.
*** Handshake has failed
GnuTLS error: Error in the certificate.

Everything other than GnuTLS talks to it ok, but git appears to use GnuTLS out of the box on Ubuntu 14.04.2 LTS so git fails with:
GIT_CURL_VERBOSE=1 git clone https://foo.example.com/some-repo.git
Cloning into 'some-repo'...
* Couldn't find host foo.example.com in the .netrc file; using defaults
* Hostname was NOT found in DNS cache
*   Trying 1.2.3.4...
* Connected to foo.example.com (1.2.3.4) port 443 (#0)
* found 173 certificates in /etc/ssl/certs/ca-certificates.crt
* server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none
* Closing connection 0
fatal: unable to access 'https://foo.example.com/some-repo.git/': server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none

We're working on fixing the incompatibility with GnuTLS, but in the mean time is there a way to force git to tell curl to use another SSL engine at runtime (i.e not rebuilding git from source)?

A:

git is built to use libcurl, libcurl is built to use a single fixed TLS library that cannot be changed in run-time.
You can however force git to use a different libcurl build at run-time, and that libcurl could be using OpenSSL. That's easiest done by putting that OpenSSL-using libcurl in a separate directory from the "standard" one you have and then you do one of these:

Make sure /etc/ld.so.conf lists that new dir before the old libcurl hosting directory - although then it'll change this setup for all programs using libcurl and you probably don't want that (based on your way of asking)
put "LD_LIBRARY_PATH=[directory where your special libcurl is] git" into a script or alias called "git2" and use that instead of the ordinary git.

