CMakeLists: Move off of modifying CMAKE_*-related flags

Modifying CMAKE_* related flags directly applies those changes to every
single CMake target. This includes even the targets we have in the
externals directory.

So, if we ever increased our warning levels, or enabled particular ones,
or enabled any other compilation setting, then this would apply to
externals as well, which is often not desirable.

This makes our compilation flag setup less error prone by only applying
our settings to our targets and leaving the externals alone entirely.

This also means we don't end up clobbering any provided flags on the
command line either, allowing users to specifically use the flags they
want.

src/CMakeLists.txt

@@ -7,19 +7,16 @@ set_property(DIRECTORY APPEND PROPERTY
 
 # Set compilation flags
 if (MSVC)
-    # Silence "deprecation" warnings
-    add_definitions(/D_CRT_SECURE_NO_WARNINGS /D_CRT_NONSTDC_NO_DEPRECATE /D_SCL_SECURE_NO_WARNINGS)
-    # Avoid windows.h junk
-    add_definitions(/DNOMINMAX)
-    # Avoid windows.h from including some usually unused libs like winsocks.h, since this might cause some redefinition errors.
-    add_definitions(/DWIN32_LEAN_AND_MEAN)
-
     set(CMAKE_CONFIGURATION_TYPES Debug Release CACHE STRING "" FORCE)
 
-    # Tweak optimization settings
-    # As far as I can tell, there's no way to override the CMake defaults while leaving user
-    # changes intact, so we'll just clobber everything and say sorry.
-    message(STATUS "Cache compiler flags ignored, please edit CMakeLists.txt to change the flags.")
+    # Silence "deprecation" warnings
+    add_definitions(-D_CRT_SECURE_NO_WARNINGS -D_CRT_NONSTDC_NO_DEPRECATE -D_SCL_SECURE_NO_WARNINGS)
+
+    # Avoid windows.h junk
+    add_definitions(-DNOMINMAX)
+
+    # Avoid windows.h from including some usually unused libs like winsocks.h, since this might cause some redefinition errors.
+    add_definitions(-DWIN32_LEAN_AND_MEAN)
 
     # /W3 - Level 3 warnings
     # /MP - Multi-threaded compilation
@@ -29,22 +26,18 @@ if (MSVC)
     # /EHsc - C++-only exception handling semantics
     # /Zc:throwingNew - let codegen assume `operator new` will never return null
     # /Zc:inline - let codegen omit inline functions in object files
-    set(CMAKE_CXX_FLAGS "/W3 /MP /Zi /Zo /permissive- /EHsc /std:c++latest /Zc:throwingNew,inline" CACHE STRING "" FORCE)
+    add_compile_options(/W3 /MP /Zi /Zo /permissive- /EHsc /std:c++latest /Zc:throwingNew,inline)
 
-    # /O2 - Optimization level 2
     # /GS- - No stack buffer overflow checks
-    # /MD - Multi-threaded runtime DLL
-    set(CMAKE_C_FLAGS_RELEASE   "/O2 /GS- /MD" CACHE STRING "" FORCE)
-    set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE}" CACHE STRING "" FORCE)
+    add_compile_options("$<$<CONFIG:Release>:/GS->")
 
     set(CMAKE_EXE_LINKER_FLAGS_DEBUG   "/DEBUG /MANIFEST:NO" CACHE STRING "" FORCE)
     set(CMAKE_EXE_LINKER_FLAGS_RELEASE "/DEBUG /MANIFEST:NO /INCREMENTAL:NO /OPT:REF,ICF" CACHE STRING "" FORCE)
 else()
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-attributes")
+    add_compile_options("-Wno-attributes")
 
     if (APPLE AND CMAKE_CXX_COMPILER_ID STREQUAL Clang)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")
-        set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -stdlib=libc++")
+        add_compile_options("-stdlib=libc++")
     endif()
 
     # Set file offset size to 64 bits.
@@ -61,8 +54,7 @@ else()
 
         if (MINGW_STATIC_BUILD)
             add_definitions(-DQT_STATICPLUGIN)
-            set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -static")
-            set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -static")
+            add_compile_options("-static")
         endif()
     endif()
 endif()