Fix Yosemite builds that use gcc-4.2 through require_gcc

The definitions that use `require_gcc` are not compatible with Apple's
clang-powered `gcc` and need gcc-4.2 from Homebrew. However, builds
using gcc-4.2 fail on Yosemite with a warning:

    couldn't understand kern.osversion `14.0.0'

Although the warning is non-fatal, the build goes to shit from there. It
seems that setting the magical value `MACOSX_DEPLOYMENT_TARGET=10.9`
makes the build work and doesn't seem to have negative consequences.

bin/ruby-build

@@ -85,6 +85,16 @@ os_information() {
   fi
 }
 
+#  9.1  -> 901
+# 10.9  -> 1009
+# 10.10 -> 1010
+osx_version() {
+  local ver="$(sw_vers -productVersion)"
+  local major="${ver%.*}"
+  local minor="${ver#*.}"
+  echo $(( major*100 + minor ))
+}
+
 build_failed() {
   { echo
     colorize 1 "BUILD FAILED"
@@ -715,6 +725,9 @@ require_gcc() {
   fi
 
   export CC="$gcc"
+  if [ "$(uname -s)" = "Darwin" ] && [ "$(osx_version)" -ge 1010 ]; then
+    export MACOSX_DEPLOYMENT_TARGET=10.9
+  fi
 }
 
 locate_gcc() {

test/compiler.bats

@@ -0,0 +1,38 @@
+#!/usr/bin/env bats
+
+load test_helper
+export MAKE=make
+
+@test "require_gcc on OS X 10.9" {
+  stub uname '-s : echo Darwin'
+  stub sw_vers '-productVersion : echo 10.9'
+  stub gcc '--version : echo 4.2.1'
+
+  run_inline_definition <<DEF
+require_gcc
+echo CC=\$CC
+echo MACOSX_DEPLOYMENT_TARGET=\${MACOSX_DEPLOYMENT_TARGET-no}
+DEF
+  assert_success
+  assert_output <<OUT
+CC=${TMP}/bin/gcc
+MACOSX_DEPLOYMENT_TARGET=no
+OUT
+}
+
+@test "require_gcc on OS X 10.10" {
+  stub uname '-s : echo Darwin'
+  stub sw_vers '-productVersion : echo 10.10'
+  stub gcc '--version : echo 4.2.1'
+
+  run_inline_definition <<DEF
+require_gcc
+echo CC=\$CC
+echo MACOSX_DEPLOYMENT_TARGET=\${MACOSX_DEPLOYMENT_TARGET-no}
+DEF
+  assert_success
+  assert_output <<OUT
+CC=${TMP}/bin/gcc
+MACOSX_DEPLOYMENT_TARGET=10.9
+OUT
+}