gcc: Fix purity

Somewhere the no-sys-dirs.patch got disabled, so gcc was looking in
/usr/local/include and /usr/lib. Since I can't fix the patch easily,
I've borrowed the --sysroot trick from clang-wrapper. This causes
builtin paths to be prefixed with /var/empty
(e.g. /var/empty/usr/lib), which don't exist.

pkgs/build-support/gcc-wrapper/gcc-wrapper.sh

@@ -77,6 +77,7 @@ if test "$NIX_ENFORCE_PURITY" = "1" -a -n "$NIX_STORE"; then
         n=$((n + 1))
     done
     params=("${rest[@]}")
+    NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE --sysroot=/var/empty"
 fi
 
 

pkgs/build-support/gcc-wrapper/ld-wrapper.sh

@@ -32,6 +32,9 @@ if test "$NIX_ENFORCE_PURITY" = "1" -a -n "$NIX_STORE" \
             # We cannot skip this; barf.
             echo "impure path \`$p' used in link" >&2
             exit 1
+        elif test "${p:0:9}" = "--sysroot"; then
+            # Our ld is not built with sysroot support (Can we fix that?)
+            :
         else
             rest=("${rest[@]}" "$p")
         fi

pkgs/development/compilers/gcc/4.8/default.nix

@@ -67,7 +67,6 @@ let version = "4.8.3";
     patches = []
       ++ optional enableParallelBuilding ./parallel-bconfig.patch
       ++ optional (cross != null) ./libstdc++-target.patch
-      # ++ optional noSysDirs ./no-sys-dirs.patch
       # The GNAT Makefiles did not pay attention to CFLAGS_FOR_TARGET for its
       # target libraries and tools.
       ++ optional langAda ./gnat-cflags.patch

pkgs/development/compilers/gcc/4.8/no-sys-dirs.patch

@@ -1,41 +0,0 @@
-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.  */
---- 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
-@@ -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)/\" \