Use CC=clang by default on Yosemite to avoid gcc-4.2

Newer MRIs will pick up gcc-4.2 from PATH and use that instead of
`/usr/bin/gcc`. While this worked up till now, it will not work in
Yosemite anymore since Homebrew's apple-gcc42 is generally not
compatible with 10.10.

So when CC has not explicitly been set, set it to `clang` to avoid
searching the PATH for any other gcc versions. This fixes MRI builds on
systems where apple-gcc42 is installed.

bin/ruby-build

@@ -479,6 +479,9 @@ build_package_standard() {
   ( if [ "${CFLAGS+defined}" ] || [ "${!PACKAGE_CFLAGS+defined}" ]; then
       export CFLAGS="$CFLAGS ${!PACKAGE_CFLAGS}"
     fi
+    if [ -z "$CC" ] && [ "$(uname -s)" = "Darwin" ] && [ "$(osx_version)" -ge 1010 ]; then
+      export CC=clang
+    fi
     ${!PACKAGE_CONFIGURE:-./configure} --prefix="${!PACKAGE_PREFIX_PATH:-$PREFIX_PATH}" \
       $CONFIGURE_OPTS ${!PACKAGE_CONFIGURE_OPTS} "${!PACKAGE_CONFIGURE_OPTS_ARRAY}" || return 1
   ) >&4 2>&1

test/compiler.bats

@@ -2,6 +2,8 @@
 
 load test_helper
 export MAKE=make
+export MAKE_OPTS='-j 2'
+export -n CFLAGS
 
 @test "require_gcc on OS X 10.9" {
   stub uname '-s : echo Darwin'
@@ -46,3 +48,37 @@ echo \$CC
 DEF
   assert_success "${TMP}/bin/gcc"
 }
+
+@test "CC=clang by default on OS X 10.10" {
+  mkdir -p "$INSTALL_ROOT"
+  cd "$INSTALL_ROOT"
+
+  stub uname '-s : echo Darwin'
+  stub sw_vers '-productVersion : echo 10.10'
+  stub cc 'false'
+  stub brew 'false'
+  stub make \
+    'echo make $@' \
+    'echo make $@'
+
+  cat > ./configure <<CON
+#!${BASH}
+echo ./configure "\$@"
+echo CC=\$CC
+echo CFLAGS=\${CFLAGS-no}
+CON
+  chmod +x ./configure
+
+  run_inline_definition <<DEF
+exec 4<&1
+build_package_standard ruby
+DEF
+  assert_success
+  assert_output <<OUT
+./configure --prefix=$INSTALL_ROOT
+CC=clang
+CFLAGS=no
+make -j 2
+make install
+OUT
+}