boost: always set toolset for known compilers

There is an edge case when cross compiling where the auto detection
script would not correctly recognize the compiler (as it is only
good at recognizing native compilers, really, which we don't have
anymore since we don't need the build->build one anymore).

If bootstrap.sh doesn't detect the compiler correctly, it'll generate
a project-config.jam with a syntax error which breaks the build in a
hard to spot way: only a warning is displayed after configuring, the
build will appear to run normally until it fails quietly just before
installing. By providing it explicitly, we can prevent this.

pkgs/development/libraries/boost/generic.nix

@@ -3,6 +3,7 @@
 , fetchpatch
 , which
 , toolset ? /**/ if stdenv.cc.isClang  then "clang"
+            else if stdenv.cc.isGNU    then "gcc"
             else null
 , enableRelease ? true
 , enableDebug ? false
@@ -193,6 +194,7 @@ stdenv.mkDerivation {
     "--libdir=$(out)/lib"
     "--with-bjam=b2" # prevent bootstrapping b2 in configurePhase
   ] ++ optional enablePython "--with-python=${python.interpreter}"
+    ++ optional (toolset != null) "--with-toolset=${toolset}"
     ++ [ (if stdenv.hostPlatform == stdenv.buildPlatform then "--with-icu=${icu.dev}" else "--without-icu") ];
 
   buildPhase = ''