gcc-cross-wrapper: fix for non-chroot build

When `libc` is not set, many lines expand to `/lib`, `/include`, which
makes gccCross use host libraries outside Nix (with a different
architecture).

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

@@ -7,7 +7,7 @@ mkdir $out/nix-support
 # Force gcc to use ld-wrapper.sh when calling ld.
 cflagsCompile="-B$out/bin/"
 
-if test -z "$nativeLibc"; then
+if test -z "$nativeLibc" -a -n "$libc"; then
     cflagsCompile="$cflagsCompile -B$gccLibs/lib -B$libc/lib/ -isystem $libc/include"
     ldflags="$ldflags -L$libc/lib"
     # Get the proper dynamic linker for glibc and uclibc.