don't build unnecessary fastlz objects

These 2 stand-alone utilities are not used by HHVM and they define a
main() that may cause certain linkers to complain about duplicate
symbols:

Linking CXX executable hhvm
duplicate symbol _main in:
    CMakeFiles/hhvm.dir/main.cpp.o
    ../../third-party/fastlz/libfastlz.a(6unpack.c.o)
duplicate symbol _usage in:
    ../../third-party/fastlz/libfastlz.a(6unpack.c.o)
    ../../third-party/fastlz/libfastlz.a(6pack.c.o)
duplicate symbol _detect_magic in:
    ../../third-party/fastlz/libfastlz.a(6unpack.c.o)
    ../../third-party/fastlz/libfastlz.a(6pack.c.o)
duplicate symbol _main in:
    CMakeFiles/hhvm.dir/main.cpp.o
    ../../third-party/fastlz/libfastlz.a(6pack.c.o)
ld: 4 duplicate symbols for architecture x86_64
Merge pull request #13 from danslo/fastlz-duplicate-symbols

don't build unnecessary fastlz objects