SDL: Add library paths of all SDL sub-packages to sdl-config

Because of how nixpkgs works, all the SDL-umbrella libraries are in
different folders. On a usual system, when using `sdl-config --libs`,
this means that SDL_gfx would also be on the linker path. Some packages
seem to depend on this being the case.

pkgs/development/libraries/SDL/find-headers.patch

@@ -1,7 +1,8 @@
-diff -ru3 SDL-1.2.15/sdl-config.in SDL-1.2.15-new/sdl-config.in
---- SDL-1.2.15/sdl-config.in	2012-01-19 10:30:06.000000000 +0400
-+++ SDL-1.2.15-new/sdl-config.in	2016-08-22 05:32:52.716397920 +0300
-@@ -42,7 +42,11 @@
+diff --git a/sdl-config.in b/sdl-config.in
+index e0fcc0c..bf7928a 100644
+--- a/sdl-config.in
++++ b/sdl-config.in
+@@ -42,14 +42,18 @@ while test $# -gt 0; do
        echo @SDL_VERSION@
        ;;
      --cflags)
@@ -13,4 +14,13 @@ diff -ru3 SDL-1.2.15/sdl-config.in SDL-1.2.15-new/sdl-config.in
 +      echo $SDL_CFLAGS @SDL_CFLAGS@
        ;;
  @ENABLE_SHARED_TRUE@    --libs)
- @ENABLE_SHARED_TRUE@      echo -L@libdir@ @SDL_RLD_FLAGS@ @SDL_LIBS@
+-@ENABLE_SHARED_TRUE@      echo -L@libdir@ @SDL_RLD_FLAGS@ @SDL_LIBS@
++@ENABLE_SHARED_TRUE@      echo -L@libdir@ @SDL_RLD_FLAGS@ @SDL_LIBS@ $SDL_LIB_PATH
+ @ENABLE_SHARED_TRUE@      ;;
+ @ENABLE_STATIC_TRUE@@ENABLE_SHARED_TRUE@    --static-libs)
+ @ENABLE_STATIC_TRUE@@ENABLE_SHARED_FALSE@    --libs|--static-libs)
+-@ENABLE_STATIC_TRUE@      echo -L@libdir@ @SDL_RLD_FLAGS@ @SDL_STATIC_LIBS@
++@ENABLE_STATIC_TRUE@      echo -L@libdir@ @SDL_RLD_FLAGS@ @SDL_STATIC_LIBS@ $SDL_LIB_PATH
+ @ENABLE_STATIC_TRUE@      ;;
+     *)
+       echo "${usage}" 1>&2

pkgs/development/libraries/SDL/setup-hook.sh

@@ -1,6 +1,7 @@
 addSDLPath () {
   if [ -e "$1/include/SDL" ]; then
     export SDL_PATH="$SDL_PATH $1/include/SDL"
+    export SDL_LIB_PATH="$SDL_LIB_PATH -L$1/lib"
   fi
 }