Fixed build errors with custom SDL location (resolves issue 774)

Uses SDL2's sdl2.m4 macros, which are installed along with SDL2. This ensures
that the correct flags are given to the compiler and linker.

platform/unix/configure.ac

@@ -34,7 +34,6 @@ AS_VAR_IF([cxx11name], [no], AC_MSG_ERROR([Can't LÖVE without C++11]), CXXFLAGS
 
 # Libraries
 AC_SEARCH_LIBS([sqrt], [m], [], AC_MSG_ERROR([Can't LÖVE without C math library]))
-AC_SEARCH_LIBS([SDL_Init], [SDL2], [], AC_MSG_ERROR([Can't LÖVE without SDL 2]))
 AC_SEARCH_LIBS([glLoadIdentity], [GL], [], AC_MSG_ERROR([Can't LÖVE without OpenGL]))
 AC_SEARCH_LIBS([alSourcePlay], [openal], [], AC_MSG_ERROR([Can't LÖVE without OpenAL]))
 AC_SEARCH_LIBS([ilInit], [IL], [], AC_MSG_ERROR([Can't LÖVE without DevIL]))
@@ -44,9 +43,11 @@ AC_SEARCH_LIBS([ModPlug_Load], [modplug], [], AC_MSG_ERROR([Can't LÖVE without
 AC_SEARCH_LIBS([ov_open], [vorbisfile], [], AC_MSG_ERROR([Can't LÖVE without VorbisFile]))
 
 # Includes
-AC_CHECK_HEADER([SDL2/SDL.h], [includes="$includes -I/usr/include/SDL2"])
 AC_CHECK_HEADER([freetype2/freetype/config/ftheader.h], [includes="$includes -I/usr/include/freetype2"])
 
+# SDL 2, treated seperately because it combines setting SDL2_LIBS and SDL2_CFLAGS into one step
+AM_PATH_SDL2([], [], AC_MSG_ERROR([Can't LÖVE without SDL 2]))
+
 # Lua, treated seperately because of --with-lua
 AC_ARG_WITH([lua], [AS_HELP_STRING([--with-lua], [Select the lua implementation])],
 			[], [with_lua=lua])

platform/unix/gen-makefile

@@ -7,6 +7,7 @@ inc_libraries="$inc_current/libraries"
 
 cat > Makefile.am << EOF
 AM_CPPFLAGS = -I$inc_current -I$inc_modules -I$inc_libraries -I$inc_libraries/enet/libenet/include \$(LOVE_INCLUDES) \$(FILE_OFFSET)
+AM_CXXFLAGS = \$(SDL_CFLAGS)
 AUTOMAKE_OPTIONS = subdir-objects
 SUBDIRS =
 
@@ -27,7 +28,7 @@ endif
 
 # libLÖVE
 lib_LTLIBRARIES = liblove.la
-liblove_la_LDFLAGS = -module -export-dynamic \$(LDFLAGS)
+liblove_la_LDFLAGS = -module -export-dynamic \$(LDFLAGS) \$(SDL_LIBS)
 liblove_la_SOURCES = \\
 EOF