BLD Add DISTDIR variable. ### Description

Adds `DISTDIR` env variable that points to the directory where libraries should be copied to when building shared libraries with recipe.

### Checklists

- [x] Add new / update outdated documentation

package:
  name: cpp-exceptions-test
  version: "0.1"
  tag:
    - core
    - pyodide.test

source:
  path: src
build:
  type: shared_library
  script: |
    em++ -c throw.cpp -o throw.o ${SIDE_MODULE_CFLAGS} -fexceptions
    em++ -c catch.cpp -o catch.o ${SIDE_MODULE_CFLAGS} -fexceptions
    mkdir dist
    em++ throw.o ${SIDE_MODULE_LDFLAGS} -o dist/cpp-exceptions-test-throw.so -fexceptions
    em++ catch.o ${SIDE_MODULE_LDFLAGS} -o dist/cpp-exceptions-test-catch.so -fexceptions

package:
  name: cpp-exceptions-test
  version: "0.1"
  tag:
    - core
    - pyodide.test

source:
  path: src
build:
  type: shared_library
  script: |
    em++ -c throw.cpp -o throw.o ${SIDE_MODULE_CFLAGS} -fexceptions
    em++ -c catch.cpp -o catch.o ${SIDE_MODULE_CFLAGS} -fexceptions
    em++ throw.o ${SIDE_MODULE_LDFLAGS} -o ${DISTDIR}/cpp-exceptions-test-throw.so -fexceptions
    em++ catch.o ${SIDE_MODULE_LDFLAGS} -o ${DISTDIR}/cpp-exceptions-test-catch.so -fexceptions
