Revert of https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=59e4c98173a79fcaa2c33253261409f38856c384
for now to fix cross fixincludes builds.

https://bugs.gentoo.org/905118
https://bugs.gentoo.org/925204
https://bugs.gentoo.org/926059
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -532,11 +532,7 @@ LINKER_PLUGIN_API_H = $(srcdir)/../include/plugin-api.h
 # Default native SYSTEM_HEADER_DIR, to be overridden by targets.
 NATIVE_SYSTEM_HEADER_DIR = @NATIVE_SYSTEM_HEADER_DIR@
 # Default cross SYSTEM_HEADER_DIR, to be overridden by targets.
-ifeq (@includedir@,$(prefix)/include)
-  CROSS_SYSTEM_HEADER_DIR = @CROSS_SYSTEM_HEADER_DIR@
-else
-  CROSS_SYSTEM_HEADER_DIR = @includedir@
-endif
+CROSS_SYSTEM_HEADER_DIR = @CROSS_SYSTEM_HEADER_DIR@
 
 # autoconf sets SYSTEM_HEADER_DIR to one of the above.
 # Purge it of unnecessary internal relative paths