Use `AM_CFLAGS` to pass custom, per-target C flags. This replaces using
`CFLAGS` which triggered this warning when running `autoreconf -fi`:
```
tests/Makefile.am:8: warning: 'CFLAGS' is a user variable, you should not override it;
tests/Makefile.am:8: use 'AM_CFLAGS' instead
```
(Only for `tests`, even though `example` and `src` also used this
method. The warning is also missing from curl, that also uses
`CFLAGS`.)
Follow-up to 3ec53f3ea2#1286Closes#1378