tests: drop useless requirements of gcc (plus testsuite tweakings)

tests: drop useless requirements of gcc (plus testsuite tweakings)

* tests/libobj14.test ($required): Remove 'gcc', as we never run
the C compiler.
* tests/targetclash.test: Likewise.
* tests/ansi6.test ($required): Remove 'gcc', as any working
C compiler should be enough.
* tests/ansi7.test: Likewise.
* tests/ansi9.test: Likewise.
* tests/ansi10.test: Likewise.
* tests/lex5.test: Likewise.
* tests/lexvpath.test: Likewise.
* tests/mmodely.test: Likewise.
* tests/pr204.test: Likewise.
* tests/pr300-lib.test: Likewise.
* tests/pr300-prog.test: Likewise.
* tests/primary3.test: Likewise.
* tests/specflg7.test: Likewise.
* tests/specflg8.test: Likewise.
* tests/subdir5.test: Likewise.
* tests/subdir8.test: Likewise.
* tests/subobj6.test: Likewise.
* tests/subst3.test: Likewise.
* tests/substre2.test: Likewise.
* tests/yacc6.test: Likewise.
* tests/yacc8.test: Likewise.
* tests/depcomp2.test: Likewise.  Also, avoid clobbering user-set
CFLAGS.
* tests/lex3.test: Likewise.
* tests/ansi3.test: Likewise.  Also, avoid 'CC=gcc' in configure.
* tests/ansi3b.test: Likewise.
* tests/ansi5.test: Likewise.
* tests/autohdr4.test ($required): Remove 'gcc', as any working
C compiler should be enough.
Also, do not reject slow dependency extractors (which we might
be forced to use now that $CC is not necessarily gcc anymore).
* tests/cond16.test ($required): Remove 'gcc', as any working
C compiler should be enough.
Since we are at it, throw in few minor tweakings (mostly cosmetic,
stylistic, or consistency-related).
* tests/cond18.test: Likewise.
* tests/cond35.test: Likewise.
* tests/gnits2.test: Likewise.
* tests/libtool3.test: Likewise.
* tests/libtool7.test: Likewise.
* tests/libtool9.test: Likewise.
* tests/ltcond.test: Likewise.
* tests/ltcond2.test: Likewise.
* tests/ltconv.test: Likewise.
* tests/ltlibsrc.test: Likewise.
* tests/nobase.test: Likewise.
* tests/nobase-libtool.test: Likewise.
* tests/pr220.test: Likewise.
* tests/pr224.test: Likewise.
* tests/pr300-ltlib.test: Likewise.
* tests/pr401.test: Likewise.
* tests/pr401b.test: Likewise.
* tests/pr401c.test: Likewise.
* tests/subpkg.test: Likewise.
* tests/target-cflags.test: Likewise.
* tests/transform.test: Likewise.
* tests/yacc4.test: Likewise.
* tests/cond19.test: Likewise.  Also, avoid clobbering user-set
CFLAGS.
* tests/cond4.test: Likewise.
* tests/depend2.test: Likewise.
* tests/pr87.test: Likewise.
* tests/subobj3.test: Likewise.
* tests/substref.test: Likewise.