cmake/target.cmake defines macros for the supported platforms, this
commit changes predefined system macros to our own macros.
__linux__ --> OS_LINUX
__APPLE__ --> OS_DARWIN
__FreeBSD__ --> OS_FREEBSD
Official docs:
Some headers from C library were deprecated in C++ and are no longer
welcome in C++ codebases. Some have no effect in C++. For more details
refer to the C++ 14 Standard [depr.c.headers] section. This check
replaces C standard library headers with their C++ alternatives and
removes redundant ones.
