add support for building on Windows with gcc

[osamasrour]

`
@echo off

set COMMON_CXXFLAGS=-o2 -Ithirdparty -Ibuild
set RGFW_CXXFLAGS=%COMMON_CXXFLAGS%
set COMMON_LIBS=user32 -lopengl32 -lgdi32
set RGFW_LIBS=%COMMON_LIBS%

mkdir build
gcc %COMMON_CXXFLAGS% -o .\build\png2c.exe src\png2c.c -Ithirdparty
call .\build\png2c.exe assets\digits.png digits > build\digits.h
call .\build\png2c.exe assets\penger_walk_sheet.png penger > build\penger_walk_sheet.h
gcc %RGFW_CXXFLAGS% -osowon src/main.c -l%RGFW_LIBS%
`