Link python*-config into VE (#460)

Required to build binaries that link against Python.
Carl Meyer, the champion for PEP 0405, has confirmed that there isn't a particular reason why `venv` doesn't do that.

bin/pyenv-virtualenv

@@ -600,6 +600,15 @@ else
   fi
 fi
 
+(
+  shopt -s nullglob
+  for extra_binary in "$PREFIX"/bin/python*-config; do
+    extra_binary_linkname="$VIRTUALENV_PATH/bin/$(basename $extra_binary)"
+    [[ -e "$extra_binary_linkname" ]] || \
+        ln -s "$extra_binary" "$extra_binary_linkname"
+  done
+)
+
 ## Create symlink in the `versions` directory for backward compatibility
 if [ -d "${VIRTUALENV_PATH}" ] && [ -n "${COMPAT_VIRTUALENV_PATH}" ]; then
   ln -fs "${VIRTUALENV_PATH}" "${COMPAT_VIRTUALENV_PATH}"

test/virtualenv.bats

@@ -29,10 +29,13 @@ unstub_pyenv() {
   stub pyenv-exec "python2.7 -m venv --help : false"
   stub pyenv-exec "python2 -m venv --help : false"
   stub pyenv-exec "python -m venv --help : false"
-  stub pyenv-exec "virtualenv * : echo PYENV_VERSION=\${PYENV_VERSION} \"\$@\""
+  stub pyenv-exec "virtualenv * : echo PYENV_VERSION=\${PYENV_VERSION} \"\$@\"; mkdir -p \"\$2/bin\""
   stub pyenv-exec "python -s -m ensurepip : false"
   stub pyenv-exec "python -s */get-pip.py : true"
   stub curl true
+  create_executable "${PYENV_VERSION}" "python-config"
+  create_executable "${PYENV_VERSION}" "python2-config"
+  create_executable "${PYENV_VERSION}" "python2.7-config"
 
   run pyenv-virtualenv "2.7.11" "venv"
 
@@ -41,8 +44,10 @@ PYENV_VERSION=2.7.11 virtualenv ${PYENV_ROOT}/versions/2.7.11/envs/venv
 Installing pip from https://bootstrap.pypa.io/pip/2.7/get-pip.py...
 rehashed
 OUT
-  assert [ -x "${PYENV_ROOT}/versions/2.7.11/envs/venv/bin/pydoc" ]
   assert_success
+  for x in pydoc python-config python2-config python2.7-config; do
+    assert [ -x "${PYENV_ROOT}/versions/2.7.11/envs/venv/bin/$x" ]
+  done
 
   unstub_pyenv
   unstub pyenv-virtualenv-prefix