configure.ac: remove "--disable-libcheck"

--disable-libcheck is a very bad idea.  If you build valgrind support
without <valgrind/memcheck.h>, then fi.h won't compile.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Merge pull request #116 from jsquyres/topic/disable-disable-libcheck

configure.ac: remove "--disable-libcheck"