Improve makefile experience for Mingw and MacOS

[DanRStevens]

The CI builds need custom variables set in the environment for Mingw and MacOS builds. Maybe we can incorporate more support directly into the makefile.

Mingw:

    WARN_EXTRA: "-Wno-redundant-decls"
    LDFLAGS_EXTRA: "-L/usr/local/x86_64-w64-mingw32/lib"

MacOS:

    echo "export CPLUS_INCLUDE_PATH=${CPLUS_INCLUDE_PATH}:$(brew --prefix)/include" >> "${BASH_ENV}"
    echo "export LDFLAGS_EXTRA=-L$(brew --prefix)/lib" >> "${BASH_ENV}"

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Additionally, for the OPHD Mingw build, the WARN_EXTRA flag also includes -Wno-dangling-reference. Though that's also used for the GCC build, and seems to be tied to a new warning flag that may be a bit buggy.

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The makefile has a TARGET_OS variable, which when set to Windows could be an indication that we expect to use the Mingw specific libraries for linking. I don't know of a good way to query for an appropriate library search path. Potentially we could scan the output of sdl2-config --libs for just the -L flag. Or we could assume it's in some standard location, such as is used by the Dockerfile. Currently the Dockerfile sets the path using a basic install prefix combined with a target architecture component:

• ARCH64=x86_64-w64-mingw32
• ARCH32=i686-w64-mingw32
• INSTALL_PREFIX=/usr/local/
• INSTALL64=${INSTALL_PREFIX}${ARCH64}/
• INSTALL32=${INSTALL_PREFIX}${ARCH32}/

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Edit: It is possible to use a Mingw specific version of pkg-config to find the needed folder.

sudo apt install mingw-w64-tools

x86_64-w64-mingw32-pkg-config --libs-only-L sdl2

-L/usr/local/x86_64-w64-mingw32/lib

This matches with the path constructed above.

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Edit 2:
A bug was discovered with the pkg-config file for the SDL2_image library, which was reported and quickly fixed:
libsdl-org/SDL_image#527 (comment)

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Edit 3:
An update to use the Mingw version of pkg-config can result in harmless warnings when the tool is not installed and TARGET_OS is set to Windows:

make: x86_64-w64-mingw32-pkg-config: No such file or directory
make: x86_64-w64-mingw32-pkg-config: No such file or directory

This can be handled the same way the warning was removed for the sdl2-config tool when it wasn't found.

SDL_CONFIG := sdl2-config
SDL_CONFIG_CFLAGS := $(shell type $(SDL_CONFIG) >/dev/null 2>&1 && $(SDL_CONFIG) --cflags)
SDL_CONFIG_LIBS := $(shell type $(SDL_CONFIG) >/dev/null 2>&1 && $(SDL_CONFIG) --libs)

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Edit 4:

It seems the Mingw version of pkg-config also has issues with the glew library:

x86_64-w64-mingw32-pkg-config --libs glew

Package glu was not found in the pkg-config search path.
Perhaps you should add the directory containing `glu.pc'
to the PKG_CONFIG_PATH environment variable
Package 'glu', required by 'glew', not found

The preprocessor flag -DGLEW_STATIC is used to disable the need for a glu library install. Hence that dependency is optional. Perhaps that package configuration file could be updated to reflect the optional nature of the dependency. Currently it's not possible to use pkg-config for the glew package without the glu dependency also installed.

Guide to pkg-config:
https://people.freedesktop.org/~dbn/pkg-config-guide.html

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The makefile has a CURRENT_OS variable, which when set to Darwin could be an indication that we expect to use brew for dependencies. That could enable us to set flags for the include search path and library search path to add the brew specific folders.

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Related:

• Issue Update Build Images #1155
• PR Update makefile to not use sdl2-config #1237
• PR Remove makefile flag -Umain for test-graphics project #1240
• PR Update Mingw build environment #1238
• PR Fix MacOS system include path for moved Brew path #1160
• PR Detect Brew path and add to system include paths #1164