Fixes sdl2 detection on unix

This patch fixes sdl2 detection and fixes this issue:
{{{
g++ -Iobj/lowlevel_Release -include obj/lowlevel_Release/precompiled.h -MMD -MP -DNDEBUG -DCONFIG_FINAL=1 -DLIB_STATIC_LINK -DUSING_PCH -I../../../source/pch/lowlevel -I../../../source -I../../../libraries/source/valgrind/include -I../../../libraries/source/cxxtest-4.4 -I/usr/X11R6/include/X11 -I/usr/X11R6/include -I/usr/include/X11 -g -Wall -O3 -Wno-switch -Wno-reorder -Wno-invalid-offsetof -Wextra -Wno-missing-field-initializers -Wunused-parameter -Wredundant-decls -Wnon-virtual-dtor -Wundef -fstack-protector-all -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -fstrict-aliasing -fno-omit-frame-pointer -fpch-preprocess -fvisibility=hidden `sdl-config --cflags` -MF obj/lowlevel_Release/cursor.d -MT "obj/lowlevel_Release/cursor.o" -o "obj/lowlevel_Release/cursor.o" -c "../../../source/lib/res/graphics/cursor.cpp"
../../../source/lib/res/graphics/cursor.cpp: In member function ‘Status SDLCursor::create(const PIVFS&, const VfsPath&, int, int)’:
../../../source/lib/res/graphics/cursor.cpp:104:63: error: ‘SDL_CreateColorCursor’ was not declared in this scope
   cursor = SDL_CreateColorCursor(surface, hotspotx_, hotspoty_);
                                                               ^
lowlevel.make:383: set di istruzioni per l'obiettivo "obj/lowlevel_Release/cursor.o" non riuscito
make[1]: *** [obj/lowlevel_Release/cursor.o] Errore 1
Makefile:60: set di istruzioni per l'obiettivo "lowlevel" non riuscito
make: *** [lowlevel] Errore 2
}}}