* src/main.c (main): Add "sanitize" to .FEATURES if ASAN is enabled.
* src/expand.c (expand_variable_output): Remember "recursive" setting
in case it's changed by the expansion of the variable.
* src/file.c (rehash_file): If we drop a file from the global 'files'
hash, remember it in rehashed_files.  We can't free it because it's
still being referenced (callers will invoke check_renamed()) but
it will be a leak since it's no longer referenced by 'files'.
* src/remake.c (update_file_1): If we drop a dependency, remember it
in dropped_list.  We can't free it because it's still being referenced
by callers but it will be a leak since it's no longer referenced as
a prerequisite.
* tests/scripts/functions/guile: Don't run Guile tests when ASAN is
enabled.
* tests/scripts/functions/wildcard: Enabling ASAN causes glob(3) to
break!  Don't run this test.
* tests/scripts/features/exec: Valgrind's exec() doesn't support
scripts with no shbang.
* tests/scripts/jobserver: Valgrind fails if TMPDIR is set to an
invalid directory: skip those tests.
* tests/scripts/features/output-sync: Ditto.
* tests/scripts/features/temp_stdin: Ditto.

Using C shells (csh, tcsh) with make is known to be problematic due
to incorrect ways it handles open file descriptors, at least.  If
the user's shell is *csh then don't try it during exec tests.

The GNU platform testers reported a number of test errors on
different systems; try to address them.

* tests/thelp.pl: A number of tests timed out with a 4-second
timeout.  Increase the default timeout to 10 seconds.
* tests/run_make_tests.pl: Executing directories on cygwin behaves
differently in Perl than make so skip these tests there.
* tests/scripts/options/symlinks: Check for the symlink feature
in make, rather than whether the system supports them.
* tests/scripts/features/implicit_search: On some systems "false"
exits with a different exit code.  Use the helper instead.
* tests/scripts/features/loadapi: Ditto.
* tests/scripts/features/output-sync: Sleep before make -f bar in
the first test as well as the second one.
* tests/scripts/features/exec: Skip on cygwin, which seems to
be "UNIX" but where scripts don't run normally.
* tests/scripts/misc/fopen-fail: Skip on cygwin, where make
eventually exits with exit code 0 and no error messages.

Avoid using posix_spawn implementations that fail asynchronously when
the spawned program can't be invoked: this means instead of getting
an error such as "No such file or directory" we get just "Exit 127".

Original implementation of the configure.ac macro provided by
Martin Dorey <martin.dorey@hds.com>

Original implementation of the regression tests provided by
Dmitry Goncharov <dgoncharov@users.sf.net>

* configure.ac: Test whether posix_spawn fails asynchronously.  In a
cross-compilation environment, assume that it does not.  If we detect
that it does, fall back to fork/exec.
* tests/scripts/features/exec: Add regression tests for different
shebang invocation methods.