add documentation, scons check for jpg support in sdl_image

It was reported on Dec 22 2014 in irc that the game is broken if
this is missing, and a search shows that bigmaps and many campaign
story screens use jpg images.

It makes more sense imo to document this as an official dependency
now, and if we decide to change everything to png's then we can go
back and disable the jpg check then.

Conflicts:
	SConstruct

INSTALL

@@ -21,7 +21,7 @@ You'll need to have these libraries and their development headers to build Wesno
  libfontconfig >= 2.4.1
  libpango (with cairo backend) >= 1.21.3
  libsdl >= 1.2.7
- libsdl-image >= 1.2 (with png support)
+ libsdl-image >= 1.2 (with png and jpg support)
  libsdl-mixer >= 1.2.12 (with Vorbis support)
  vorbisfile
  libsdl-net

SConstruct

@@ -357,7 +357,9 @@ if env["prereqs"]:
         conf.CheckSDL("SDL_mixer", require_version = '1.2.12') and \
         conf.CheckLib("vorbisfile") and \
         conf.CheckSDL("SDL_image", require_version = '1.2.0') and \
-        conf.CheckOgg() or Warning("Client prerequisites are not met. wesnoth, cutter and exploder cannot be built.")
+        conf.CheckOgg() and \
+        conf.CheckPNG() and \
+        conf.CheckJPG() or Warning("Client prerequisites are not met. wesnoth, cutter and exploder cannot be built.")
 
     have_X = False
     if have_client_prereqs:

scons/sdl.py

@@ -122,6 +122,37 @@ def CheckPNG(context):
         context.Result("no")
         return False
 
+def CheckJPG(context):
+    test_program = '''
+    #include <SDL_image.h>
+    #include <stdlib.h>
+
+    int main(int argc, char **argv)
+    {
+            SDL_RWops *src;
+            char *testimage = "data/core/images/maps/background.jpg";
+
+            src = SDL_RWFromFile(testimage, "rb");
+            if (src == NULL) {
+                    exit(2);
+            }
+            exit(!IMG_isJPG(src));
+    }
+\n
+'''
+    context.Message("Checking for JPG support in SDL... ")
+    if context.env["host"]:
+        context.Result("n/a (cross-compile)")
+        return True
+    (result, output) = context.TryRun(test_program, ".c")
+    if result:
+        context.Result("yes")
+        return True
+    else:
+        context.Result("no")
+        return False
+
 config_checks = { 'CheckSDL' : CheckSDL,
                   'CheckOgg' : CheckOgg,
-                  'CheckPNG' : CheckPNG }
+                  'CheckPNG' : CheckPNG,
+                  'CheckJPG' : CheckJPG }