for generating hw/xfree86/loader/sdksyms.c the following command line is used:
CPP='$(CPP)' AWK='$(AWK)' $(srcdir)/sdksyms.sh $(top_srcdir) $(AM_CFLAGS) $(CFLAGS) $(INCLUDES)

however when sdksyms.c is combiled $(CPPFLAGS) are added as well.
I am cross-compiling and use CPPFLAGS="-isystem <mysysroot>/usr/include -isystem <mysysroot>/include"

"-I<mysysroot>/usr/include" is also added via pkg-config results. Without the CPPFLAGS the search order is messed up and <mysysroot>/usr/include/os.h" is used instead of <xorg-server>/include/os.h
as a result I get
sdksyms.c:1072: error: 'limitDataSpace' undeclared here (not in a function)
sdksyms.c:1073: error: 'limitStackSpace' undeclared here (not in a function)
sdksyms.c:1074: error: 'limitNoFile' undeclared here (not in a function)
make[5]: *** [sdksyms.lo] Error 1

I'm not sure what the correct commandline to generate sdksyms.c is but CPPFLAGS should be added.