Taken from downstream:
http://anonscm.debian.org/gitweb/?p=pkg-xorg/lib/mesa.git;a=blob;f=debian/patches/14-fix-osmesa-build.diff;h=
00581d0e1833c5492d9050e1bf3d5e658cad782e;hb=refs/heads/ubuntu%2B1
v2: Move the added line immediately after -I$(top_srcdir)/src/mapi
NOTE: This is a candidate for the 9.1 and 9.0 branches.
Acked-by: Kenneth Graunke <kenneth@whitecape.org> (v1)
Reviewed-by: Matt Turner <mattst88@gmail.com>
AM_CPPFLAGS = \
-I$(top_srcdir)/include \
-I$(top_srcdir)/src/mapi \
+ -I$(top_builddir)/src/mapi \
-I$(top_srcdir)/src/mesa/ \
$(DEFINES) \
$(API_DEFINES)