Commit a2cd8a03 moved the includes in python/Makefile.am from INCLUDES to AM_CPPFLAGS. This change breaks the build when using --with-python.

This happens because automake does not use AM_CPPFLAGS for tagets which have a target-specific *_CPPFLAGS variable. And at least when using automake-1.12.3 on Gentoo, a basic definition of libxsltmod_la_CPPFLAGS automatically gets added python/Makefile.am:

libxsltmod_la_CPPFLAGS = -shared

As a result, the rule for building libxsltmod_la-libxslt.lo gets expanded to

libxsltmod_la-libxslt.lo: libxslt.c
	$(AM_V_CC)$(LIBTOOL) $(AM_V_lt) --tag=CC $(AM_LIBTOOLFLAGS) $(LIBTOOLFLAGS) --mode=compile $(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(libxsltmod_la_CPPFLAGS) $(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS) -c -o libxsltmod_la-libxslt.lo `test -f 'libxslt.c' || echo '$(srcdir)/'`libxslt.c

and the build fails with

/bin/sh ../libtool  --tag=CC   --mode=compile x86_64-pc-linux-gnu-gcc -DHAVE_CONFIG_H -I. -I..  -shared  -I/usr/include/libxml2 -march=native -O2 -pipe -Wall -c -o libxsltmod_la-libxslt.lo `test -f 'libxslt.c' || echo './'`libxslt.c
libtool: compile:  x86_64-pc-linux-gnu-gcc -DHAVE_CONFIG_H -I. -I.. -I/usr/include/libxml2 -march=native -O2 -pipe -Wall -c libxslt.c  -fPIC -DPIC -o .libs/libxsltmod_la-libxslt.o
libxslt.c:14:20: fatal error: Python.h: No such file or directory
compilation terminated.

The build failure can be avoided by appending the includes to libxsltmod_la_CPPFLAGS instead of assigning to AM_CPPFLAGS.