Use clang instead of gcc on OS X 10.9

Apple replaced gcc with clang, but kept around a stub gcc to call
clang. This adds a check in `configure` to detect when gcc is
really clang, and switch to using clang in this situation.

Closes #10811.

Author: erickt

configure

@@ -549,6 +549,19 @@ then
     err "either clang or gcc is required"
 fi
 
+# OS X 10.9, gcc is actually clang. This can cause some confusion in the build
+# system, so if we find that gcc is clang, we should just use clang directly.
+if [ $CFG_OSTYPE = apple-darwin -a -z "$CFG_ENABLE_CLANG" ]
+then
+    CFG_OSX_GCC_VERSION=$("$CFG_GCC" --version 2>&1 | grep "Apple LLVM version")
+    if [ $? -eq 0 ]
+    then
+        step_msg "on OS X 10.9, forcing use of clang"
+        CFG_ENABLE_CLANG=1
+        putvar CFG_ENABLE_CLANG
+    fi
+fi
+
 if [ ! -z "$CFG_LLVM_ROOT" -a -e "$CFG_LLVM_ROOT/bin/llvm-config" ]
 then
     step_msg "using custom LLVM at $CFG_LLVM_ROOT"