Suggestion: change CMakeLists.txt to build on MacOS 10.5

[Betonmischer86]

Building Lost Mission on a PowerPC Mac (MacOS 10.5) failed with the following errors:

cc1plus: error: unrecognized command line option "-ffp-contract=off"
cc1plus: error: unrecognized command line option "-Wno-strict-overflow"

The first one I fixed following advice on the same issue with Dhewm3 by changing "add_compile_options(-ffp-contract=off)" in CMakeLists.txt to:

CHECK_CXX_COMPILER_FLAG("-ffp-contract=off" cxx_has_fp-contract)
if(cxx_has_fp-contract)
add_compile_options(-ffp-contract=off)
endif()

I also replaced "add_compile_options(-Wno-strict-overflow)" with:

CHECK_CXX_COMPILER_FLAG("-Wno-strict-overflow" cxx_has_Wno-strict-overflow)
if(cxx_has_Wno-strict-overflow)
add_compile_options(-Wno-strict-overflow)
endif()

Now I'm able to build d3le.dylib and run the mod.