Avoid build failure on Python bindings when GDAL CPPFLAGS are proparagated by Debian build environment, defining GDAL_COMPILATION flag

git-svn-id: https://svn.osgeo.org/gdal/trunk@28124 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx