modify build_detect_platform to check for OS X Mavericks

On Mavericks clang is the default compiler, so modify build_detect_platform
to detect that OS X version and set the necessary compiler and linker
switches.

Drop the -fno-builtin-memcmp compiler option for Mavericks, since the clang
compiler doesn't honor it and just warns about it instead. Measurements
with the db_bench test program (built here with "make check") show that
using clang's -fno-builtin option decreases performance slightly, so it
isn't used either.

Successfully tested on Mavericks, Mountain Lion, and Lion.