GNUmakefile: Simplify further #86

Job	Run time
build (C distribution build, -O2 -Wall -W -std=c90 -Wdeclaration-after-statement -Werror)	2m 31s
build (C distribution build (clang), -O2 -Wall -W -std=c90 -Wdeclaration-after-statement -Werror,...	2m 33s
build (C, y, -O2 -Wall -W -std=c99 -Werror)	2m 35s
build (C (clang), y, -O2 -Wall -W -std=c99 -Werror, clang)	3m 15s
build (java, java, javac, -Xlint:all -Werror)	3m 2s
build (go_old, ubuntu-20.04, golang-1.13, go)	3m 9s
build (go_new, golang-1.20, go)	3m 53s
build (javascript_node, node, nodejs)	3m 3s
build (rust, rust, rustc)	3m 41s
build (csharp, mcs, mono-devel)	2m 50s
build (Pascal, fpc, fpc)	3m 33s
build (Python 3.8, python3.8, ubuntu-20.04, python3.8, 1)	20m 59s
build (Python 3.10, python3.10, ubuntu-22.04, python3.10, 1)	19m 6s
build (Python (pypy3), pypy3, pypy3)	4m 43s
build (Ada, gprbuild, gnat gprbuild)	3m 9s
build (Windows (C), windows-latest, y, sccache)	3m 13s
build (Windows (Go), windows-latest, go, mingw32-make, mingw-w64-ucrt-x86_64-go, sccache)	5m 32s
	1h 30m 47s

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