CMake: simplify some expressions

[Tilka]

• use STREQUAL instead of MATCHES when not checking a regex
• use WIN32/APPLE instead of manually checking CMAKE_SYSTEM_NAME
• remove some duplicate parentheses

[BhaaLseN]

Are the ${CMAKE_SYSTEM_NAME} STREQUAL "Linux" ok like that? ie. does the system name always spell "Linux" and exactly that on Linux, or could it possibly include kernel/version numbers?

And: is there no equivalent identifier for Linux, like there is for WIN32, APPLE and ANDROID?

[Tilka]

From the CMake documentation:

On systems that have the uname command, this variable is set to the output of uname -s.

And no, there's no LINUX variable, only UNIX.

[waddlesplash]

LGTM