gcc7: make building w/LTO support optional, don't use in bootstrap tools

pkgs/development/compilers/gcc/7/default.nix

@@ -6,6 +6,7 @@
 , profiledCompiler ? false
 , staticCompiler ? false
 , enableShared ? true
+, enableLTO ? true
 , texinfo ? null
 , perl ? null # optional, for texi2pod (then pod2man)
 , gmp, mpfr, libmpc, gettext, which
@@ -257,7 +258,7 @@ stdenv.mkDerivation ({
 
     # Basic configuration
     [
-      "--enable-lto"
+      (if enableLTO then "--enable-lto" else "--disable-lto")
       "--disable-libstdcxx-pch"
       "--without-included-gettext"
       "--with-system-zlib"