tests: don't require GNU make where it's not strictly needed

tests: don't require GNU make where it's not strictly needed

* tests/autohdr3.test: Rewrite to be stricter when make is GNU
make.  Drop the requirement of GNU make.
* tests/aclocal5.test: Drop GNU make requirement, it's not truly
needed.  Add reference to similar tests `remake-subdir*.test'.
* remake-subdir-gnu.test, remake-subdir-from-subdir.test,
remake-subdir.test, remake-subdir2.test: Add reference to
each other, and to related test `aclocal5.test'.
* tests/aclocal6.test: Drop GNU make requirement, it's not
truly needed.
* tests/confh6.test: Likewise.
* tests/lex3.test: Likewise.
* tests/remake11.test: Likewise.
* tests/subdir5.test: Likewise.
* tests/subdir8.test: Likewise.
* tests/werror2.test: Likewise.
* tests/conff.test: Likewise, and ensure verbose printing of
captured make output.
* tests/lex5.test: Tweak so that GNU make is no more required.
* tests/version7.test: Likewise.
* tests/maken2.test: Add explicative comment for why this test
required GNU make.
* tests/maken4.test: Let it run also with BSD makes supporting
the `.MAKE' special target.
* tests/output6.test: Use proper m4 quoting in configure.in.
Expand make macros with one-character name using `$(x)', not
`$x', for portability.  Move checks in the makefiles, rather
than relying on grepping the output from make.  Drop the now
unneeded GNU make requirement.