Add CMake build system

This should hopefully work torwards compatibility with more systems.
I've tried to make this as general as possible but some small assumptions
about the compiler are made. I've also tried to recreate the build process
as closely as possible, but I had to change some things slightly to work
with CMake (version strings, mainly).

For now, it doesn't allow in-source builds, as that could overwrite the
Makefile.

This adds:

- Support for more build systems
- Automatic dependency generation
- Performance gains (especially when using i.e. Ninja)

Defaults to Release build.

src/CMakeLists.txt

@@ -0,0 +1,99 @@
+#
+# This file is part of RGBDS.
+#
+# Copyright (c) 2020 RGBDS contributors.
+#
+# SPDX-License-Identifier: MIT
+#
+
+set(common_src
+    "extern/err.c"
+    "extern/getopt.c"
+    "version.c"
+    )
+
+BISON_TARGET(ASMy "asm/asmy.y"
+             "${PROJECT_SOURCE_DIR}/src/asm/asmy.c"
+             DEFINES_FILE "${PROJECT_SOURCE_DIR}/src/asm/asmy.h"
+             )
+
+# Lexer is not present yet
+if(False) # FLEX_FOUND
+  FLEX_TARGET(Lexer "asm/lexer.l"
+              "${PROJECT_SOURCE_DIR}/src/asm/lexer.c"
+              )
+  ADD_FLEX_BISON_DEPENDENCY(Lexer ASMy)
+  set(Lexer_SOURCE "${FLEX_Lexer_OUTPUTS}")
+else()
+  set(Lexer_SOURCE "asm/lexer.c")
+endif()
+
+set(rgbasm_src
+    "${BISON_ASMy_OUTPUT_SOURCE}"
+    "${Lexer_SOURCE}"
+    "asm/charmap.c"
+    "asm/fstack.c"
+    "asm/globlex.c"
+    "asm/macro.c"
+    "asm/main.c"
+    "asm/math.c"
+    "asm/output.c"
+    "asm/rpn.c"
+    "asm/section.c"
+    "asm/symbol.c"
+    "asm/util.c"
+    "asm/warning.c"
+    "extern/utf8decoder.c"
+    "hashmap.c"
+    "linkdefs.c"
+    )
+
+set(rgbfix_src
+    "fix/main.c"
+    )
+
+set(rgbgfx_src
+    "gfx/gb.c"
+    "gfx/main.c"
+    "gfx/makepng.c"
+    )
+
+set(rgblink_src
+    "link/assign.c"
+    "link/main.c"
+    "link/object.c"
+    "link/output.c"
+    "link/patch.c"
+    "link/script.c"
+    "link/section.c"
+    "link/symbol.c"
+    "hashmap.c"
+    "linkdefs.c"
+    )
+
+foreach(PROG "asm" "fix" "gfx" "link")
+  add_executable(rgb${PROG}
+                 ${rgb${PROG}_src}
+                 ${common_src}
+                 )
+endforeach()
+
+if(CMAKE_VERSION VERSION_LESS 2.8.12)
+  add_definitions(${PNG_DEFINITIONS})
+  include_directories(${PNG_INCLUDE_DIRS})
+  target_link_libraries(rgbgfx ${PNG_LIBRARIES})
+else()
+  target_compile_definitions(rgbgfx PRIVATE ${PNG_DEFINITIONS})
+  target_include_directories(rgbgfx PRIVATE ${PNG_INCLUDE_DIRS})
+  target_link_libraries(rgbgfx PRIVATE ${PNG_LIBRARIES})
+endif()
+
+include(CheckLibraryExists)
+check_library_exists("m" "sin" "" HAS_LIBM)
+if(HAS_LIBM)
+  if(CMAKE_VERSION VERSION_LESS 2.8.12)
+    target_link_libraries(rgbasm LINK_PRIVATE "m")
+  else()
+    target_link_libraries(rgbasm PRIVATE "m")
+  endif()
+endif()