feature: C89 adherence #72

Job	Run time
Build (5.1, windows-latest)	55s
MSYS2 Build (mingw64, x86_64, 5.4, lua, lua)	45s
Build (5.1, ubuntu-latest)	17s
Build C89 (5.1.5)	17s
MSYS2 Build (mingw64, x86_64, 5.3, lua53, lua5.3)	50s
Build C++ (5.1.5)	20s
Build (5.1, macos-latest)	16s
Build C89 (5.2.4)	24s
MSYS2 Build (mingw64, x86_64, 5.1, lua51, lua5.1)	49s
Build C++ (5.2.4)	43s
Build (5.2, windows-latest)	41s
Build C89 (5.3.6)	17s
MSYS2 Build (mingw64, x86_64, 5.1, luajit, luajit)	48s
Build C++ (5.3.6)	27s
Build (5.2, ubuntu-latest)	13s
Build C89 (5.4.7)	27s
MSYS2 Build (ucrt64, ucrt-x86_64, 5.4, lua, lua)	48s
Build C++ (5.4.7)	20s
Build (5.2, macos-latest)	18s
MSYS2 Build (ucrt64, ucrt-x86_64, 5.3, lua53, lua5.3)	44s
MSYS2 Build (ucrt64, ucrt-x86_64, 5.1, lua51, lua5.1)	45s
MSYS2 Build (ucrt64, ucrt-x86_64, 5.1, luajit, luajit)	45s
MSYS2 Build (clang64, clang-x86_64, 5.4, lua, lua)	54s
MSYS2 Build (clang64, clang-x86_64, 5.3, lua53, lua5.3)	55s
Build (5.3, windows-latest)	25s
Build (5.3, ubuntu-latest)	19s
MSYS2 Build (clang64, clang-x86_64, 5.1, lua51, lua5.1)	52s
MSYS2 Build (clang64, clang-x86_64, 5.1, luajit, luajit)	50s
Build (5.3, macos-latest)	18s
Build (5.4, windows-latest)	25s
Build (5.4, ubuntu-latest)	20s
Build (5.4, macos-latest)	17s
Build (luajit, windows-latest)	1m 9s
Build (luajit, ubuntu-latest)	15s
Upload rockspec	0s
	19m 8s

Provide feedback