Use a variable for `PKG_CONFIG` for distros that cross-compile.

Fixes #450.

Change-Id: If399caf0566f85b1e20e6ea3fedfc524018d2e6c
Reviewed-on: https://code-review.googlesource.com/c/re2/+/61950
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed