cmake: disable optimization on debug builds

While our debug builds on MSVC platforms already tune the code optimizer
to aid debugging code, all the other platforms still use the default
optimization level. This makes it hard for developers on these platforms
to actually debug code while maintaining his sanity due to optimizations
like inlined code, elided variables etc.

To help this common use case, we can simply follow the MSVC example and
turn off code optimization with "-O0" for debug builds. While it would
be preferable to instead use "-Og" supported by more modern compilers,
we cannot guarantee that this level is available on all supported
platforms.

CMakeLists.txt

@@ -478,7 +478,7 @@ ELSE ()
 		SET(CMAKE_C_FLAGS "-std=c99 -D_POSIX_C_SOURCE=200112L -D__EXTENSIONS__ -D_POSIX_PTHREAD_SEMANTICS ${CMAKE_C_FLAGS}")
 	ENDIF()
 
-	SET(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -D_DEBUG")
+	SET(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -D_DEBUG -O0")
 
 	IF (MINGW OR MSYS) # MinGW and MSYS always do PIC and complain if we tell them to
 		STRING(REGEX REPLACE "-fPIC" "" CMAKE_SHARED_LIBRARY_C_FLAGS "${CMAKE_SHARED_LIBRARY_C_FLAGS}")