Fix cmake build for git submodules on unix

[crvarner]

CMake build when including SDL_image as git submodule was broken for unix:

$ make
[  1%] Generating pnglibconf.c
[  2%] Generating pnglibconf.out
/home/cvarner/workspace/game-template/build/external/SDL_image/external/libpng-1.6.37/pnglibconf.c:34:11: fatal error: zlib.h: No such file or directory
   34 | # include <zlib.h>
      |           ^~~~~~~~
compilation terminated.
CMake Error at scripts/genout.cmake:78 (message):
  Failed to generate
  /home/cvarner/workspace/game-template/build/external/SDL_image/external/libpng-1.6.37/pnglibconf.out.tf1

looking at the scripts/genout.cmake created by cmake, you can see how the includes are generated

set(INCDIR "/home/cvarner/workspace/game-template/build/external/SDL_image/external/libpng-1.6.37")
...
set(ZLIBINCDIR "external/zlib-1.2.11")    <--- this comes from ZLIB_INCLUDE_DIR
...
  set(INCLUDES "-I${INCDIR}")
  if(ZLIBINCDIR)
    foreach(dir ${ZLIBINCDIR})
      list(APPEND INCLUDES "-I${dir}")
    endforeach()
  endif()

which results in the following compile commands while compiling the generated file pnglibconf.c:

cd /home/cvarner/workspace/game-template/build/external/SDL_image/external/libpng-1.6.37
/usr/bin/cc' '-E'
'-I/home/cvarner/workspace/game-template/external/SDL_image/external/libpng-1.6.37'
'-I/home/cvarner/workspace/game-template/build/external/SDL_image/external/libpng-1.6.37'
'-I/home/cvarner/workspace/game-template/build/external/SDL_image/external/libpng-1.6.37'
'-Iexternal/zlib-1.2.11'
'-DPNGLIB_LIBNAME=PNG16_0'
'-DPNGLIB_VERSION=1.6.37'
'-DSYMBOL_PREFIX='
'-DPNG_NO_USE_READ_MACROS'
'-DPNG_BUILDING_SYMBOL_TABLE'
'/home/cvarner/workspace/game-template/build/external/SDL_image/external/libpng-1.6.37/pnglibconf.c

Note that the relative include "external/zlib-1.2.11" resolves to /home/cvarner/workspace/game-template/external/SDL_image/external/libpng-1.6.37/external/zlib-1.2.11 instead of the zlib source, causing the original error.

Updating the top level CMakeLists.txt to use an absolute path results in the following compile command, which is better

'/usr/bin/cc'
'-E'
'-I/home/cvarner/workspace/game-template/external/SDL_image/external/libpng-1.6.37'
'-I/home/cvarner/workspace/game-template/build/external/SDL_image/external/libpng-1.6.37'
'-I/home/cvarner/workspace/game-template/build/external/SDL_image/external/libpng-1.6.37'
'-I/home/cvarner/workspace/game-template/external/SDL_image/external/zlib-1.2.11'
'-DPNGLIB_LIBNAME=PNG16_0' '-DPNGLIB_VERSION=1.6.37' '-DSYMBOL_PREFIX=' '-DPNG_NO_USE_READ_MACROS' '-DPNG_BUILDING_SYMBOL_TABLE'
'/home/cvarner/workspace/game-template/build/external/SDL_image/external/libpng-1.6.37/pnglibconf.c'

We are still missing zlib's output directory though so we get the following error:

/home/cvarner/workspace/game-template/external/SDL_image/external/zlib-1.2.11/zlib.h:34:10: fatal error: zconf.h: No such file or directory
   34 | #include "zconf.h"
      |          ^~~~~~~~~
compilation terminated.
CMake Error at scripts/genout.cmake:79 (message):
  Failed to generate
  /home/cvarner/workspace/game-template/build/external/SDL_image/external/libpng-1.6.37/pnglibconf.out.tf1

zlibs include directories should include both the original source and the binary dir. This PR does that with 2 changes:

1. CMake: including current src directory into include path madler/zlib#218 is a long standing zlib issue with a known solution CMake: use CMAKE_CURRENT_SOURCE_DIR, *not* top-level src dir. Fixes #218 madler/zlib#219 that only appears to remain unfixed because the repo hasn't been updated since 2017.

2. With (1) fixed, ZLIB_INCLUDE_DIR can be set using the INCLUDE_DIRECTORIES property of the zlib target created by adding the zlib subdirectory. This variable now correctly contains all include directories resulting in the following compile command:

'/usr/bin/cc' '-E'
'-I/home/cvarner/workspace/game-template/external/SDL_image_fork/external/libpng-1.6.37'
'-I/home/cvarner/workspace/game-template/build/external/SDL_image_fork/external/libpng-1.6.37'
'-I/home/cvarner/workspace/game-template/build/external/SDL_image_fork/external/libpng-1.6.37'
'-I/home/cvarner/workspace/game-template/build/external/SDL_image_fork/external/zlib-1.2.11'
'-I/home/cvarner/workspace/game-template/external/SDL_image_fork/external/zlib-1.2.11'
'-DPNGLIB_LIBNAME=PNG16_0'
'-DPNGLIB_VERSION=1.6.37'
'-DSYMBOL_PREFIX='
'-DPNG_NO_USE_READ_MACROS'
'-DPNG_BUILDING_SYMBOL_TABLE'
'/home/cvarner/workspace/game-template/build/external/SDL_image_fork/external/libpng-1.6.37/pnglibconf.c'

The build succeeds with no issues from there on

[sezero]

Noticed this one lat, sorry. Applying.