Fix mruby build

Copy build_config.rb to build directory because now
build_config.rb.lock is created along side it.

Define toolchain depending on CC environment variable.

third-party/Makefile.am

@@ -54,7 +54,10 @@ EXTRA_DIST += build_config.rb mruby/*
 .PHONY: all-local clean mruby
 
 mruby:
-	MRUBY_CONFIG="${srcdir}/build_config.rb" \
+	mkdir -p "${abs_builddir}/mruby/build"
+	diff "${srcdir}/build_config.rb" "${abs_builddir}/mruby/build/build_config.rb" >& /dev/null || \
+	cp "${srcdir}/build_config.rb" "${abs_builddir}/mruby/build"
+	MRUBY_CONFIG="${abs_builddir}/mruby/build/build_config.rb" \
 	BUILD_DIR="${abs_builddir}/mruby/build" \
 	INSTALL_DIR="${abs_builddir}/mruby/build/install/bin" \
 	CC="${CC}" CXX="$(firstword $(CXX))" LD="${LD}" \
@@ -66,7 +69,7 @@ mruby:
 all-local: mruby
 
 clean-local:
-	MRUBY_CONFIG="${srcdir}/build_config.rb" \
+	MRUBY_CONFIG="${abs_builddir}/mruby/build/build_config.rb" \
 	BUILD_DIR="${abs_builddir}/mruby/build" \
 	"${srcdir}/mruby/minirake" -f "${srcdir}/mruby/Rakefile" clean
 

third-party/build_config.rb

 MRuby::Build.new do |conf|
-  # TODO use same compilers configured in configure script
-  toolchain :clang
+  toolchain :clang if ENV['CC'].include? "clang"
+  toolchain :gcc if ENV['CC'].include? "gcc"
 
   # C++ project needs this.  Without this, mruby exception does not
   # properly destory C++ object allocated on stack.