modify Makefile to be more in-line with the GNU standard.

Mainly so the makefile is a bit less esoteric to use, debugging flags
must be specified manually.

changed:
- `$RM` for calling `rm`
- removed unused `$ISWIN`
- renamed `test` to `check`
- added `install` and `install-strip` recipes, which for now remain
empty
- added `-g` to `$CFLAGS`, since it helps debugging, and in case of
failures makes bug report less of a headache.

I decided to not modify it further so "specialised tools" don't need to
store files in the git repo, which seemed to complicate logic.
This is the same reason to why I chose to not adapt clean in a way to
introduce `mostlyclean`.

.github/workflows/ci.yaml

@@ -54,4 +54,4 @@ jobs:
         if: steps.cache-deps.outputs.cache-hit != 'true' && runner.os == 'Windows'
 
       - run: make -j all
-      - run: make -j test
+      - run: make -j check