Make __FreeBSD_cc_version predefined macro configurable at build time

[dim]

Summary

The FreeBSDTargetInfo class has always set the __FreeBSD_cc_version predefined macro to a rather static value, calculated from the major OS version.

In the FreeBSD base system, we would like to start incrementing the value of this macro whenever we make any signifant change to clang, so we need a way to configure the macro's value at build time.

I am proposing to use FREEBSD_CC_VERSION for this, which we can define using the -D command line option in the FreeBSD build system. Stock builds will still keep the earlier value.

[emaste]

LGTM assuming the #ifndef style is OK

[dim]

@bkramer, @rsmith, is this an acceptable way of getting this macro in?

[dim]

Belatedly adding cfe-commits.

cfe/trunk/lib/Basic/Targets.cpp

DragonFlyBSDTargetInfo(const llvm::Triple &Triple, const TargetOptions &Opts)
     case llvm::Triple::x86:
     case llvm::Triple::x86_64:
       this->MCountName = ".mcount";
       break;
     }
   }
 };
 
+#ifndef FREEBSD_CC_VERSION
+#define FREEBSD_CC_VERSION 0U
+#endif
+
 // FreeBSD Target
 template<typename Target>
 class FreeBSDTargetInfo : public OSTargetInfo<Target> {
 protected:
   void getOSDefines(const LangOptions &Opts, const llvm::Triple &Triple,
                     MacroBuilder &Builder) const override {
     // FreeBSD defines; list based off of gcc output
 
     unsigned Release = Triple.getOSMajorVersion();
     if (Release == 0U)
-      Release = 8;
+      Release = 8U;
+    unsigned CCVersion = FREEBSD_CC_VERSION;
+    if (CCVersion == 0U)
+      CCVersion = Release * 100000U + 1U;
 
     Builder.defineMacro("__FreeBSD__", Twine(Release));
-    Builder.defineMacro("__FreeBSD_cc_version", Twine(Release * 100000U + 1U));
+    Builder.defineMacro("__FreeBSD_cc_version", Twine(CCVersion));
     Builder.defineMacro("__KPRINTF_ATTRIBUTE__");
     DefineStd(Builder, "unix", Opts);
     Builder.defineMacro("__ELF__");
 
     // On FreeBSD, wchar_t contains the number of the code point as
     // used by the character set of the locale. These character sets are
     // not necessarily a superset of ASCII.
     //