cmake: set "-D_DEBUG" on non-Windows platforms

In our code base, we have some occasions where we use the "_DEBUG"
preprocessor macro to enable additional code which should not be part of
release builds. While we define this flag on MSVC platforms, it is
guarded by the conditional `WIN32 AND NOT CYGWIN` on other platforms
since 19be3f9e (Improve MSVC compiler, linker flags, 2013-02-13). While
this condition can be fulfilled by the MSVC platform, it is never
encountered due to being part of the `ELSE` part of `IF (MSVC)`.

The intention of the conditional was most likely to avoid the
preprocessor macro on Cygwin platforms, but to include it on everthing
else. As such, the correct condition here would be `IF (NOT CYGWIN)`
instead. But digging a bit further, the condition is only ever used in
two places:

1. To skip the test in "core::structinit", which should also work on
   Cygwin.
2. In "src/win32/git2.rc", where it is used to set additional file
   flags. As this file is included in MSVC builds only, it cannot cause
   any harm to set "_DEBUG" on Cygwin here.

As such, we can simply drop the conditional and always set "-D_DEBUG" on
all platforms.

CMakeLists.txt

@@ -478,9 +478,7 @@ ELSE ()
 		SET(CMAKE_C_FLAGS "-std=c99 -D_POSIX_C_SOURCE=200112L -D__EXTENSIONS__ -D_POSIX_PTHREAD_SEMANTICS ${CMAKE_C_FLAGS}")
 	ENDIF()
 
-	IF (WIN32 AND NOT CYGWIN)
-		SET(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -D_DEBUG")
-	ENDIF ()
+	SET(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -D_DEBUG")
 
 	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}")