[gimp-macos-build] Fix libxml2 build

[Alex Samorukov <asamorukov src gnome org>]

commit 3a411c9a75a7d0c2d6e160bb6d5e716a801d405c
Author: Oleksii Samorukov <oleksii_samorukov mckinsey com>
Date:   Sun Oct 27 17:57:36 2019 +0100

    Fix libxml2 build

 jhbuildrc-gtk-osx-gimp | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)
---
diff --git a/jhbuildrc-gtk-osx-gimp b/jhbuildrc-gtk-osx-gimp
index 77525cc..069d7c5 100644
--- a/jhbuildrc-gtk-osx-gimp
+++ b/jhbuildrc-gtk-osx-gimp
@@ -157,9 +157,8 @@ environ_prepend('OBJCFLAGS', "-O3")
 # use our modulset
 moduleset = 'https://gitlab.gnome.org/samm-git/gtk-osx/raw/fork-test/modulesets-stable/gtk-osx.modules'
 
-module_extra_env['libxml2'] = { "PYTHON": os.path.expanduser("python3.6") }
-
-# And more...
+# force python 3.6 for the libxml2 to avoid build issues
+module_extra_env['libxml2'] = { "ac_cv_path_PYTHON": "python3.6" }
 
 class _cmd_bootstrap_gtk_osx_gimp(jhbuild.commands.base.cmd_build):
     doc = 'Build buildsystem tools not provided by MacOS.'