mesa: Replace a priori knowledge of gcc builtins with configure tests.

Presumbly this will let clang and other compilers use the built-ins as
well.

Notice two changes specifically:
   - in _mesa_next_pow_two_64(), always use __builtin_clzll and add a
     static assertion that this is safe.
   - in macros.h, remove the clang-specific definition since it should
     be able to detect __builtin_unreachable in configure.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com> [C bits]
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

scons/gallium.py

@@ -583,6 +583,24 @@ def generate(env):
             env.Append(CCFLAGS = ['-fopenmp'])
             env.Append(LIBS = ['gomp'])
 
+    if gcc_compat:
+        ccversion = env['CCVERSION']
+        cppdefines += [
+            'HAVE___BUILTIN_EXPECT',
+            'HAVE___BUILTIN_FFS',
+            'HAVE___BUILTIN_FFSLL',
+        ]
+        if distutils.version.LooseVersion(ccversion) >= distutils.version.LooseVersion('3.4'):
+            cppdefines += [
+                'HAVE___BUILTIN_CTZ',
+                'HAVE___BUILTIN_POPCOUNT',
+                'HAVE___BUILTIN_POPCOUNTLL',
+                'HAVE___BUILTIN_CLZ',
+                'HAVE___BUILTIN_CLZLL',
+            ]
+        if distutils.version.LooseVersion(ccversion) >= distutils.version.LooseVersion('4.5'):
+            cppdefines += ['HAVE___BUILTIN_UNREACHABLE']
+
     # Load tools
     env.Tool('lex')
     env.Tool('yacc')