Unix: CMakeLists.txt compat fixes, suppress fewer warnings

`LIST(APPEND ..)` adds a `;` to the gcc command line, replaced with string
interpolation.

Only use "-Wno-address-of-temporary" on macOS (=clang),
on Linux/gcc use "-fpermissive" which still shows the compiler warning
so it can be fixed later.

CMakeLists.txt

@@ -14,14 +14,19 @@ PROJECT(Descent3)
 IF (UNIX)
 SET (D3_GAMEDIR "~/Descent3/")
 
+  IF (APPLE)
+    SET(BITS "-m64")
+    SET(EXTRA_CXX_FLAGS "-Wno-address-of-temporary")
+  ELSE()
+    SET(BITS "-m32")
+    SET(EXTRA_CXX_FLAGS "-fpermissive")
+  ENDIF()
+
   SET(CMAKE_CXX_COMPILER "g++")
-  SET(CMAKE_CXX_FLAGS "-O0 -g -Wno-write-strings -Wno-multichar -Wno-address-of-temporary")
-  SET(CMAKE_C_FLAGS "-O0 -g")
+  SET(CMAKE_CXX_FLAGS "-O0 -g -Wno-write-strings -Wno-multichar ${BITS} ${EXTRA_CXX_FLAGS}")
+  SET(CMAKE_C_FLAGS "-O0 -g ${BITS}")
   SET(CMAKE_C_COMPILER "gcc")
-  IF (NOT APPLE)
-    LIST(APPEND CMAKE_CXX_FLAGS "-m32")
-    LIST(APPEND CMAKE_C_FLAGS "-m32")
-  ENDIF()
+
   FIND_PACKAGE( SDL REQUIRED )
   IF (APPLE)
     # Provide FIND_PACKAGE( SDL_image ) below with an include dir and library that work with brew-installed sdl2_image