feature: C89 adherence #69

Job	Run time
Build (5.1, windows-latest)	33s
MSYS2 Build (mingw64, x86_64, 5.4, lua, lua)	46s
Build C++ (5.1.5)	39s
Build (5.1, ubuntu-latest)	21s
Build C89 (5.1.5)	24s
MSYS2 Build (mingw64, x86_64, 5.3, lua53, lua5.3)	44s
Build C++ (5.2.4)	18s
Build (5.1, macos-latest)	17s
Build C89 (5.2.4)	17s
MSYS2 Build (mingw64, x86_64, 5.1, lua51, lua5.1)	48s
Build C++ (5.3.6)	18s
Build (5.2, windows-latest)	32s
MSYS2 Build (mingw64, x86_64, 5.1, luajit, luajit)	54s
Build C89 (5.3.6)	20s
MSYS2 Build (ucrt64, ucrt-x86_64, 5.4, lua, lua)	43s
Build C++ (5.4.7)	19s
Build (5.2, ubuntu-latest)	11s
Build C89 (5.4.7)	21s
MSYS2 Build (ucrt64, ucrt-x86_64, 5.3, lua53, lua5.3)	43s
Build (5.2, macos-latest)	17s
MSYS2 Build (ucrt64, ucrt-x86_64, 5.1, lua51, lua5.1)	46s
Build (5.3, windows-latest)	39s
MSYS2 Build (ucrt64, ucrt-x86_64, 5.1, luajit, luajit)	44s
Build (5.3, ubuntu-latest)	19s
MSYS2 Build (clang64, clang-x86_64, 5.4, lua, lua)	53s
Build (5.3, macos-latest)	15s
MSYS2 Build (clang64, clang-x86_64, 5.3, lua53, lua5.3)	53s
Build (5.4, windows-latest)	46s
MSYS2 Build (clang64, clang-x86_64, 5.1, lua51, lua5.1)	54s
Build (5.4, ubuntu-latest)	11s
MSYS2 Build (clang64, clang-x86_64, 5.1, luajit, luajit)	55s
Build (5.4, macos-latest)	21s
Build (luajit, windows-latest)	34s
Build (luajit, ubuntu-latest)	19s
Upload rockspec	0s
	18m 14s

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