Newer gcc (5.2 and up) won't compile older gcc because they added an 'extension'
and made their code depnd on it, and then decided it had been a bad idea and
ripped it out again but older gcc goes "oh of course newer gcc will support
this forever". The <strike>Aristrocrats!</strike> FSF!

diff -ruN gcc-core/gcc/toplev.c gcc-core.bak/gcc/toplev.c
--- gcc-core/gcc/toplev.c	2006-10-09 11:27:14.000000000 -0500
+++ gcc-core.bak/gcc/toplev.c	2015-12-15 16:19:26.643494852 -0600
@@ -531,7 +531,7 @@
    for floor_log2 and exact_log2; see toplev.h.  That construct, however,
    conflicts with the ISO C++ One Definition Rule.   */
 
-#if GCC_VERSION < 3004 || !defined (__cplusplus)
+#if 0
 
 /* Given X, an unsigned number, return the largest int Y such that 2**Y <= X.
    If X is 0, return -1.  */
diff -ruN gcc-core/gcc/toplev.h gcc-core.bak/gcc/toplev.h
--- gcc-core/gcc/toplev.h	2006-02-26 14:23:40.000000000 -0600
+++ gcc-core.bak/gcc/toplev.h	2015-12-15 17:47:59.755637132 -0600
@@ -151,12 +151,6 @@
 /* Return true iff flags are set as if -ffast-math.  */
 extern bool fast_math_flags_set_p	(void);
 
-/* Return log2, or -1 if not exact.  */
-extern int exact_log2                  (unsigned HOST_WIDE_INT);
-
-/* Return floor of log2, with -1 for zero.  */
-extern int floor_log2                  (unsigned HOST_WIDE_INT);
-
 /* Inline versions of the above for speed.  */
 #if GCC_VERSION >= 3004
 # if HOST_BITS_PER_WIDE_INT == HOST_BITS_PER_LONG
@@ -170,13 +164,13 @@
 #  define CTZ_HWI __builtin_ctz
 # endif
 
-extern inline int
+static inline int
 floor_log2 (unsigned HOST_WIDE_INT x)
 {
   return x ? HOST_BITS_PER_WIDE_INT - 1 - (int) CLZ_HWI (x) : -1;
 }
 
-extern inline int
+static inline int
 exact_log2 (unsigned HOST_WIDE_INT x)
 {
   return x == (x & -x) && x ? (int) CTZ_HWI (x) : -1;