Improve feature detection.

Add V8_GLIBC_PREREQ() macro to test the version of the GNU C library.
Make V8_GNUC_PREREQ() work for compilers that masquerade as GCC (you
can always use V8_CC_* to test for a specific one if you need to).
Add V8_LIBC_* to detect the C library in use.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/23717009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16450 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix typo breaking Mac build after commit r16450.

TBR=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/23522010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16451 ce2b1a6d-e550-0410-aec6-3dcde31c8c00