[PATCH] Fix build failure due to incorrect include ordering

Commit 627da3d8 made an incorrect change: INCLUDES should have been changed to AM_CPPFLAGS rather than its contents being moved to AM_CFLAGS. Putting preprocessor flags in AM_CFLAGS results in them being passed in the wrong order relative to CPPFLAGS, which results in build failure when a previous version of libshout is installed and its headers are in the same place as those of its dependencies.