attempt add include dir

Author: cvanelteren

.github/workflows/build.yml

@@ -67,6 +67,7 @@ jobs:
             CPATH={project}/extern/include
             LIBRARY_PATH={project}/extern/lib
             LD_LIBRARY_PATH={project}/extern/lib
+            CFLAGS="-I{project}/extern/include"
             SETUPTOOLS_USE_DISTUTILS=stdlib
           CIBW_TEST_COMMAND: >
             cd {project} && python -m pytest --cov="mpl_toolkits.basemap"