diff -ru gcc-4.3.1-orig/gcc/cppdefault.c gcc-4.3.1/gcc/cppdefault.c
--- gcc-4.3.1-orig/gcc/cppdefault.c	2007-07-26 10:37:01.000000000 +0200
+++ gcc-4.3.1/gcc/cppdefault.c	2008-06-25 17:48:23.000000000 +0200
@@ -41,6 +41,10 @@
 # undef CROSS_INCLUDE_DIR
 #endif
 
+#undef LOCAL_INCLUDE_DIR
+#undef SYSTEM_INCLUDE_DIR
+#undef STANDARD_INCLUDE_DIR
+
 const struct default_include cpp_include_defaults[]
 #ifdef INCLUDE_DEFAULTS
 = INCLUDE_DEFAULTS;
diff -ru gcc-4.3.1-orig/gcc/gcc.c gcc-4.3.1/gcc/gcc.c
--- gcc-4.3.1-orig/gcc/gcc.c	2008-03-02 23:55:19.000000000 +0100
+++ gcc-4.3.1/gcc/gcc.c	2008-06-25 17:52:53.000000000 +0200
@@ -1478,10 +1478,10 @@
 /* Default prefixes to attach to command names.  */
 
 #ifndef STANDARD_STARTFILE_PREFIX_1
-#define STANDARD_STARTFILE_PREFIX_1 "/lib/"
+#define STANDARD_STARTFILE_PREFIX_1 ""
 #endif
 #ifndef STANDARD_STARTFILE_PREFIX_2
-#define STANDARD_STARTFILE_PREFIX_2 "/usr/lib/"
+#define STANDARD_STARTFILE_PREFIX_2 ""
 #endif
 
 #ifdef CROSS_DIRECTORY_STRUCTURE  /* Don't use these prefixes for a cross compiler.  */
@@ -1515,8 +1515,8 @@
 /* For native compilers, these are well-known paths containing
    components that may be provided by the system.  For cross
    compilers, these paths are not used.  */
-static const char *const standard_exec_prefix_1 = "/usr/libexec/gcc/";
-static const char *const standard_exec_prefix_2 = "/usr/lib/gcc/";
+static const char *const standard_exec_prefix_1 = "/no-such-path/";
+static const char *const standard_exec_prefix_2 = "/no-such-path/";
 static const char *md_exec_prefix = MD_EXEC_PREFIX;
 static const char *md_startfile_prefix = MD_STARTFILE_PREFIX;
 static const char *md_startfile_prefix_1 = MD_STARTFILE_PREFIX_1;
diff -ru gcc-4.3.1-orig/gcc/Makefile.in gcc-4.3.1/gcc/Makefile.in
--- gcc-4.3.1-orig/gcc/Makefile.in	2008-05-11 20:54:15.000000000 +0200
+++ gcc-4.3.1/gcc/Makefile.in	2008-06-25 17:48:23.000000000 +0200
@@ -378,7 +378,11 @@
 MD5_H	    = $(srcdir)/../include/md5.h
 
 # Default native SYSTEM_HEADER_DIR, to be overridden by targets.
-NATIVE_SYSTEM_HEADER_DIR = /usr/include
+# Nix: we override NATIVE_SYSTEM_HEADER_DIR in order to prevent
+# `fixinc' from fixing header files in /usr/include.  However,
+# NATIVE_SYSTEM_HEADER_DIR must point to an existing directory, so set
+# it to some dummy directory.
+NATIVE_SYSTEM_HEADER_DIR = $(NIX_FIXINC_DUMMY)
 # Default cross SYSTEM_HEADER_DIR, to be overridden by targets.
 CROSS_SYSTEM_HEADER_DIR = @CROSS_SYSTEM_HEADER_DIR@
 
@@ -3277,7 +3281,7 @@
   -DGPLUSPLUS_INCLUDE_DIR=\"$(gcc_gxx_include_dir)\" \
   -DGPLUSPLUS_TOOL_INCLUDE_DIR=\"$(gcc_gxx_include_dir)/$(target_noncanonical)\" \
   -DGPLUSPLUS_BACKWARD_INCLUDE_DIR=\"$(gcc_gxx_include_dir)/backward\" \
-  -DLOCAL_INCLUDE_DIR=\"$(local_includedir)\" \
+  -DLOCAL_INCLUDE_DIR=\"/no-such-dir\" \
   -DCROSS_INCLUDE_DIR=\"$(CROSS_SYSTEM_HEADER_DIR)\" \
   -DTOOL_INCLUDE_DIR=\"$(gcc_tooldir)/include\" \
   -DPREFIX=\"$(prefix)/\" \