Fix and split recent testsuite addition for compile/link flags.

Fix and split recent testsuite addition for compile/link flags.

* tests/flags.at (passing flags through libtool): Split into ...
(passing CC flags through libtool)
(passing CXX flags through libtool)
(passing F77 flags through libtool)
(passing FC flags through libtool)
(passing GCJ flags through libtool): ... these five tests,
factorized with m4_foreach.  Fix F77 and FC compile and link
commands; add GCJ tag.  Use LT_AT_TAG to correctly skip tags
for which no compiler exists.  Use $EXEEXT where appropriate.
Drop unneeded use of reload_cmds.
Reports by Peter Rosin and Peter O'Gorman.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>