Use tcmalloc for gcc

LTO builds are a few percent faster with tcmalloc

XXX numbers

and the memory usage is similar:

tcmalloc also has better heap profiling capabilities.

This patch makes gcc build with tcmalloc when it is available.

gcc/:

2014-09-19  Andi Kleen  <ak@linux.intel.com>

	* configure.ac: Link with tcmalloc when available.
	* configure: Regenerate.