KERN_BOOTTIME is defined inside sys/sysctl.h.
Fix typo, it existed there a long time causing configure test to always fail.
Set GCC-specific flags before most configure checks.

This fixes detection of isfinite() and other C99 functions in Solaris 10
and other platforms.
Remove AM_PROG_CC_C_O, deprecated since automake 1.14.
Merge pull request #1805 from jimis/master

configure.ac reordering