tests: don't try to prevent flex from including unistd.h.

tests: don't try to prevent flex from including unistd.h.

This patch is from https://bugs.gnu.org/59994.

* t/cond35.sh: This patch is related to an effort to prepare
Automake for future GCC/Clang versions which set c99 as default
standard to be used.

In the current version of flex, not having unistd.h leads to
implicit function declarations that are not compatible with C99.

A future version of flex will (hopefully) make the "NO_UNISTD"
flag a no-op, and include unistd.h by default.

Also, while flex-dedicated tests had this macro set,
yacc tests did not, despite their use of flex.

For consistency, all the yacc/flex related tests now have the
"--never-interactive" option set in AM_LFLAGS, and do not try
define an isatty function.

* t/lex-clean-cxx.sh: Likewise.
* t/lex-clean.sh: Likewise.
* t/lex-depend-cxx.sh: Likewise.
* t/lex-depend.sh: Likewise.
* t/lex-header.sh: Likewise.
* t/lex-lib-external.sh: Likewise.
* t/lex-lib.sh: Likewise.
* t/lex-libobj.sh: Likewise.
* t/lex-line.sh: Likewise.
* t/lex-multiple.sh: Likewise.
* t/lex-nodist.sh: Likewise.
* t/lex-noyywrap.sh: Likewise.
* t/lex-pr204.sh: Likewise.
* t/lex-subobj-nodep.sh: Likewise.
* t/lex3.sh: Likewise.
* t/lex5.sh: Likewise.
* t/lexvpath.sh: Likewise.
* t/silent-lex.sh: Likewise.
* t/silent-many-languages.sh: Likewise.
* t/silent-yacc-headers.sh: Likewise.
* t/silent-yacc.sh: Likewise.
* t/subpkg-yacc.sh: Likewise.
* t/suffix10.tap: Likewise.
* t/yacc-auxdir.sh: Likewise.
* t/yacc-basic.sh: Likewise.
* t/yacc-bison-skeleton-cxx.sh: Likewise.
* t/yacc-bison-skeleton.sh: Likewise.
* t/yacc-clean-cxx.sh: Likewise.
* t/yacc-clean.sh: Likewise.
* t/yacc-cxx-grepping.sh: Likewise.
* t/yacc-cxx.sh: Likewise.
* t/yacc-d-basic.sh: Likewise.
* t/yacc-d-cxx.sh: Likewise.
* t/yacc-d-vpath.sh: Likewise.
* t/yacc-deleted-headers.sh: Likewise.
* t/yacc-depend.sh: Likewise.
* t/yacc-depend2.sh: Likewise.
* t/yacc-dist-nobuild-subdir.sh: Likewise.
* t/yacc-dist-nobuild.sh: Likewise.
* t/yacc-dry.sh: Likewise.
* t/yacc-grepping.sh: Likewise.
* t/yacc-grepping2.sh: Likewise.
* t/yacc-headers-and-dist-pr47.sh: Likewise.
* t/yacc-line.sh: Likewise.
* t/yacc-misc.sh: Likewise.
* t/yacc-mix-c-cxx.sh: Likewise.
* t/yacc-nodist.sh: Likewise.
* t/yacc-pr204.sh: Likewise.
* t/yacc-subdir.sh: Likewise.
* t/yacc-vpath.sh: Likewise.
* t/yacc-weirdnames.sh: Likewise.
* t/yflags-cmdline-override.sh: Likewise.
* t/yflags-cxx.sh: Likewise.
* t/yflags-d-false-positives.sh: Likewise.
* t/yflags-force-conditional.sh: Likewise.
* t/yflags-force-override.sh: Likewise.
* t/yflags-var-expand.sh: Likewise.
* t/yflags.sh: Likewise.