Make precompiled headers cross buildable.

http://bugs.gentoo.org/show_bug.cgi?id=262912

diff -urN gcc-4.5.0.orig//libstdc++-v3/include/Makefile.am gcc-4.5.0/libstdc++-v3/include/Makefile.am
--- gcc-4.5.0.orig//libstdc++-v3/include/Makefile.am	2010-07-27 11:22:34.000000000 +0200
+++ gcc-4.5.0/libstdc++-v3/include/Makefile.am	2010-07-27 11:21:29.000000000 +0200
@@ -893,7 +893,7 @@
 	${pch1_output_builddir} ${pch2_output_builddir} ${pch3_output_builddir}
 pch_output_anchors = \
 	${pch1_output_anchor} ${pch2_output_anchor} ${pch3_output_anchor}
-PCHFLAGS=-x c++-header $(CXXFLAGS)
+PCHFLAGS=-x c++-header $(CXXFLAGS) -nostdinc++
 if GLIBCXX_BUILD_PCH
 pch_build = ${pch_output}
 else
diff -urN gcc-4.5.0.orig//libstdc++-v3/include/Makefile.in gcc-4.5.0/libstdc++-v3/include/Makefile.in
--- gcc-4.5.0.orig//libstdc++-v3/include/Makefile.in	2010-04-02 20:18:06.000000000 +0200
+++ gcc-4.5.0/libstdc++-v3/include/Makefile.in	2010-07-27 11:21:47.000000000 +0200
@@ -1119,7 +1119,7 @@
 pch_output_anchors = \
 	${pch1_output_anchor} ${pch2_output_anchor} ${pch3_output_anchor}
 
-PCHFLAGS = -x c++-header $(CXXFLAGS)
+PCHFLAGS = -x c++-header $(CXXFLAGS) -nostdinc++
 @GLIBCXX_BUILD_PCH_FALSE@pch_build = 
 @GLIBCXX_BUILD_PCH_TRUE@pch_build = ${pch_output}