CMake build system refactoring

[Mc-Zen]

CMakeLists.txt

• Now we add_subdirectory() the max-sdk-base.
• Add a version (0.6.0, based on the latest release on github).
• Replace the custom API target with an interface target and set the version. Also add the include directories for this target in order to enable appending the min-api include path by linking to this target.
• Add interface target min-api-test-headers (used for unit tests).
• Move option "Treat warnings as errors" from min-posttarget.cmake to the main CMakeLists.txt.

c74_add_min_target.cmake

• Replacement function c74_add_min_target() for min-pretarget.cmake/min-posttarget.cmake
• Calls c74_add_max_target() and creates a library target as well as a project and appends some additional definitions.
• In contrast to min-pre/posttarget.cmake: omitted duplicated configuration code which is already called from max-sdk-base (no need to set C++ standard (done by c74_add_max_target()); no need to check xcode version (done in CMakeLists.txt); no need to check OSX architectures (done in c74_add_max_target()))
• Common code between this file and min-posttarget.cmake for setting warning flags on mac has been refactored to a separate file.

min-posttarget.cmake

• Moved option "Treat warnings as errors" to CMakeLists.txt.
• Replaced settings warnings flags on Mac (see above).

For uniformity:
Should/can we rename the API and the LIB library targets to min-api and min-lib to match the repo name and therefore also give them a slightly more descriptive name (an alias to the old name can be created)?