MATH: Just look for a GCC compatible compiler for __builtin_clz()

There's no need to check for GCC 3.4+ anymore, since we always require
a C++11 compiler.  Just look for __GNUC__, since this is an old
builtin and Clang always has it too.

Also #undef the LT() temporary macro while there, so that it can't
pollute something else by accident, since it's a short macro name in
a header file.

common/math.h

@@ -66,7 +66,7 @@ struct Complex {
 	float re, im;
 };
 
-#if GCC_ATLEAST(3, 4)
+#if defined(__GNUC__)
 inline int intLog2(uint32 v) {
 	// This is a slightly optimized implementation of log2 for natural numbers
 	// targeting gcc. It also saves some binary size over our fallback
@@ -94,6 +94,7 @@ static const char LogTable256[256] = {
 	-1, 0, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3,
 	LT(4), LT(5), LT(5), LT(6), LT(6), LT(6), LT(6),
 	LT(7), LT(7), LT(7), LT(7), LT(7), LT(7), LT(7), LT(7)
+#undef LT
 };
 
 inline int intLog2(uint32 v) {