Moving to -std=gnu11 (another case for requiring GCC >= 4.7)

[Joseph Myers]

What do people think of moving glibc to build with -std=gnu11 (the default 
in GCC 5 and later) instead of -std=gnu99?

Note 1: -std=gnu99 is hardcoded in Makeconfig.  GCC 4.6 only has 
-std=gnu1x not -std=gnu11.  So to support 4.6 we'd either have to hardcode 
building with the deprecated -std=gnu1x name (which doesn't really seem a 
good idea) or add a configure test.

Note 2: libm-test.inc currently uses the macros __FLT_DENORM_MIN__ etc.; 
using -std=gnu11 would allow moving to the standard versions FLT_TRUE_MIN 
etc. (which GCC 4.6 has in float.h, but only when compiling in C11 mode, 
not for -std=gnu99).  I'd also like to use the macros FLT_DECIMAL_DIG etc. 
(which again require C11 mode to use the standard versions rather than the 
GCC predefines).

-- 
Joseph S. Myers
joseph@codesourcery.com