Let llvm and clang use same GCC as Pyston used.

Pyston will try use the gcc which found in system path to compile
itself. But llvm and clang will use the gcc in /usr/bin dir by
default. This different will cause inconsistency in some situation.
So add -DGCC_INSTALL_PREFIX to let clang use same version of gcc as
Pyston used.

Makefile

@@ -75,6 +75,7 @@ TESTS_DIR := $(abspath ./test/tests)
 
 GPP := g++
 GCC := gcc
+HOST_GCC=$ENV{GCC_PATH}
 
 ifeq ($(V),1)
 	VERBOSE := 1
@@ -656,7 +657,7 @@ CMAKE_SETUP_GCC := $(CMAKE_DIR_GCC)/build.ninja
 $(CMAKE_SETUP_GCC):
 	@$(MAKE) cmake_check
 	@mkdir -p $(CMAKE_DIR_GCC)
-	cd $(CMAKE_DIR_GCC); CC='$(GCC)' CXX='$(GPP)' cmake $(COMMON_CMAKE_OPTIONS) -DCMAKE_BUILD_TYPE=Debug
+	cd $(CMAKE_DIR_GCC); CC='$(GCC)' CXX='$(GPP)' cmake $(COMMON_CMAKE_OPTIONS) -DCMAKE_BUILD_TYPE=Debug -DGCC_INSTALL_PREFIX=$(HOST_GCC)
 .PHONY: pyston_gcc
 pyston_gcc: $(CMAKE_SETUP_GCC)
 	$(NINJA) -C $(CMAKE_DIR_GCC) pyston copy_stdlib $(CMAKE_SHAREDMODS) ext_cpython $(NINJAFLAGS)