modules: make FindSDL2.cmake work with MinGW version 2.0.5 and higher.

Why nobody told me *all these years*?

doc/changelog.dox

@@ -197,6 +197,9 @@ See also:
 
 -   @ref building-packages-msys "MSYS2 packages" are now in official
     repositories, installable directly via `pacman`
+-   `FindSDL2.cmake` was updated to work with MinGW version 2.0.5 and newer,
+    since these dropped the old directory hierarchy. Older versions have both
+    the original and the new hierarchy, so it should work with these as well.
 -   Updated *extremely* outdated Emscripten boilerplate from the
     [base-emscripten bootstrap project](https://github.com/mosra/magnum-bootstrap/tree/base-emscripten)
     (the HTML markup was unchanged since 2013, heh)

modules/FindSDL2.cmake

@@ -45,11 +45,29 @@
 if(CORRADE_TARGET_EMSCRIPTEN)
     set(_SDL2_PATH_SUFFIXES SDL)
 else()
-    # Precompiled libraries for Windows are in x86/x64 subdirectories
-    if(CMAKE_SIZEOF_VOID_P EQUAL 8)
-        set(_SDL_LIBRARY_PATH_SUFFIX lib/x64)
-    elseif(CMAKE_SIZEOF_VOID_P EQUAL 4)
-        set(_SDL_LIBRARY_PATH_SUFFIX lib/x86)
+    set(_SDL2_PATH_SUFFIXES SDL2)
+    if(WIN32)
+        # Precompiled libraries for MSVC are in x86/x64 subdirectories
+        if(MSVC)
+            if(CMAKE_SIZEOF_VOID_P EQUAL 8)
+                set(_SDL2_LIBRARY_PATH_SUFFIX lib/x64)
+            elseif(CMAKE_SIZEOF_VOID_P EQUAL 4)
+                set(_SDL2_LIBRARY_PATH_SUFFIX lib/x86)
+            endif()
+
+        # Both includes and libraries for MinGW are in some directory deep
+        # inside. There's also a CMake config file but it has HARDCODED path
+        # to /opt/local/i686-w64-mingw32, which doesn't make ANY SENSE,
+        # especially on Windows.
+        elseif(MINGW)
+            if(CMAKE_SIZEOF_VOID_P EQUAL 8)
+                set(_SDL2_LIBRARY_PATH_SUFFIX x86_64-w64-mingw32/lib)
+                list(APPEND _SDL2_PATH_SUFFIXES x86_64-w64-mingw32/include/SDL2)
+            elseif(CMAKE_SIZEOF_VOID_P EQUAL 4)
+                set(_SDL2_LIBRARY_PATH_SUFFIX i686-w64-mingw32/lib)
+                list(APPEND _SDL2_PATH_SUFFIXES i686-w64-mingw32/include/SDL2)
+            endif()
+        endif()
     endif()
 
     find_library(SDL2_LIBRARY_RELEASE
@@ -58,15 +76,14 @@ else()
         # the dylib first so it is preferred. Not sure how this maps to debug
         # config though :/
         NAMES SDL2-2.0 SDL2
-        PATH_SUFFIXES ${_SDL_LIBRARY_PATH_SUFFIX})
+        PATH_SUFFIXES ${_SDL2_LIBRARY_PATH_SUFFIX})
     find_library(SDL2_LIBRARY_DEBUG
         NAMES SDL2d
-        PATH_SUFFIXES ${_SDL_LIBRARY_PATH_SUFFIX})
+        PATH_SUFFIXES ${_SDL2_LIBRARY_PATH_SUFFIX})
     # FPHSA needs one of the _DEBUG/_RELEASE variables to check that the
     # library was found -- using SDL_LIBRARY, which will get populated by
     # select_library_configurations() below.
     set(SDL2_LIBRARY_NEEDED SDL2_LIBRARY)
-    set(_SDL2_PATH_SUFFIXES SDL2)
 endif()
 
 include(SelectLibraryConfigurations)