More configure tricks

I previously set up all of the libs needed to compile Project Mod on my Windows XP VirtualBox instance, but it takes forever to compile, so I'm trying once again to set up cross compiling from Linux. It turns out that lots of libs will compile just fine if you pass in a few flags when you configure them:

./configure --host=i586-mingw32msvc --prefix=/usr/i586-mingw32msvc

Of course, more will go wrong, so you have to be willing to dive into the configure script and search for override flags. Some are easier, like how SDL_net requires --disable-sdltest. Without that flag the configure script will try to run a test program, which will fail since it's not producing Linux executables. Others are a bit trickier:

ac_cv_lib_png_png_create_read_struct=yes ./configure --host=i586-mingw32msvc --prefix=/usr/i586-mingw32msvc --disable-sdltest

SDL_image needs libpng for PNG image support, but it compiles a test program to check if libpng works. I know that libpng is fine (I just compiled it), so I made it skip the check by defining_that_long_environment_variable. This command makes the configure script spit out:

checking png.h usability... yes
checking png.h presence... yes
checking for png.h... yes
checking for png_create_read_struct in -lpng... (cached) yes

Without the environment variable that last line fails and PNG support is left out.