Propagate Build, Compiler, Linker, and OS X Settings to External Projects

Propagates the following to external projects:

`CMAKE_BUILD_TYPE`
`CMAKE_C_COMPILER`
`CMAKE_C_FLAGS`
`CMAKE_CXX_COMPILER`
`CMAKE_CXX_FLAGS`
`CMAKE_EXE_LINKER_FLAGS` (if necessary)
`CMAKE_SHARED_LINKER_FLAGS` (if necessary)
`CMAKE_OSX_ARCHITECTURES` (if Apple)
`CMAKE_OSX_DEPLOYMENT_TARGET` (if Apple)
`CMAKE_OSX_SYSROOT` (if Apple)

Tested on OS X 10.8.3 with Apple C/C++ Compiler 4.2 and CMake 2.8.10.