FindSDL: Make FindSDL_*.cmake path suffixes consistent with main module

Add the 'SDL' path suffix to the FindSDL_*.cmake modules.  Add a comment
explaining the purpose of suffixes as in commit v3.0.0-rc1~540^2
(FindSDL: Add path suffixes for <prefix>/include/..., 2013-10-08).
Inspired-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

Modules/FindSDL_image.cmake

@@ -54,7 +54,9 @@ find_path(SDL_IMAGE_INCLUDE_DIR SDL_image.h
   HINTS
     ENV SDLIMAGEDIR
     ENV SDLDIR
-  PATH_SUFFIXES include/SDL include/SDL12 include/SDL11 include
+  PATH_SUFFIXES SDL
+                # path suffixes to search inside ENV{SDLDIR}
+                include/SDL include/SDL12 include/SDL11 include
 )
 
 if(CMAKE_SIZEOF_VOID_P EQUAL 8)

Modules/FindSDL_mixer.cmake

@@ -54,7 +54,9 @@ find_path(SDL_MIXER_INCLUDE_DIR SDL_mixer.h
   HINTS
     ENV SDLMIXERDIR
     ENV SDLDIR
-  PATH_SUFFIXES include/SDL include/SDL12 include/SDL11 include
+  PATH_SUFFIXES SDL
+                # path suffixes to search inside ENV{SDLDIR}
+                include/SDL include/SDL12 include/SDL11 include
 )
 
 if(CMAKE_SIZEOF_VOID_P EQUAL 8)

Modules/FindSDL_net.cmake

@@ -54,7 +54,9 @@ find_path(SDL_NET_INCLUDE_DIR SDL_net.h
   HINTS
     ENV SDLNETDIR
     ENV SDLDIR
-  PATH_SUFFIXES include/SDL include/SDL12 include/SDL11 include
+  PATH_SUFFIXES SDL
+                # path suffixes to search inside ENV{SDLDIR}
+                include/SDL include/SDL12 include/SDL11 include
 )
 
 if(CMAKE_SIZEOF_VOID_P EQUAL 8)

Modules/FindSDL_sound.cmake

@@ -98,7 +98,9 @@ find_path(SDL_SOUND_INCLUDE_DIR SDL_sound.h
   HINTS
     ENV SDLSOUNDDIR
     ENV SDLDIR
-  PATH_SUFFIXES include/SDL include/SDL12 include/SDL11 include
+  PATH_SUFFIXES SDL
+                # path suffixes to search inside ENV{SDLDIR}
+                include/SDL include/SDL12 include/SDL11 include
   )
 
 find_library(SDL_SOUND_LIBRARY

Modules/FindSDL_ttf.cmake

@@ -54,7 +54,9 @@ find_path(SDL_TTF_INCLUDE_DIR SDL_ttf.h
   HINTS
     ENV SDLTTFDIR
     ENV SDLDIR
-  PATH_SUFFIXES include/SDL include/SDL12 include/SDL11 include
+  PATH_SUFFIXES SDL
+                # path suffixes to search inside ENV{SDLDIR}
+                include/SDL include/SDL12 include/SDL11 include
 )
 
 if(CMAKE_SIZEOF_VOID_P EQUAL 8)