Detect and set Xcode 6.3 clang version

This becomes necessary to avoid compilation errors with
Xcode 7 and clang 7.0.0. (Note that its version information
doesn't state which LLVM version it's based on, though we
suspect it could be 3.7.0svn.)

Change-Id: I2bfc7f2b73ca7a61798b123cc2715037028f0e5f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qcompilerdetection.h

@@ -155,7 +155,9 @@
 /* Clang also masquerades as GCC */
 #    if defined(__apple_build_version__)
 #      /* http://en.wikipedia.org/wiki/Xcode#Toolchain_Versions */
-#      if __apple_build_version__ >= 6000051
+#      if __apple_build_version__ >= 6020049
+#        define Q_CC_CLANG 306
+#      elif __apple_build_version__ >= 6000051
 #        define Q_CC_CLANG 305
 #      elif __apple_build_version__ >= 5030038
 #        define Q_CC_CLANG 304