posix: improve portability of constant definitions.

Rather than try to catalog what compiler/OS releases supply what
constant symbols in their headers with a rats nest of `#if __BSD`
etc, wrap all of the contentious preprocessor defines in matching
`#ifdefs` so that they are only provided by luaposix in the
environments that define them.  This should provide much better
future portability to new compiler/OS combinations.
* lposix.c (lua_open_posix_c): Simplify the declarations by using
a consistent MENTRY definition.
Except for extremely well-known groups of symbols, and groups
that are already inside _POSIX_VERSION >= 200112L guards, wrap
each constant declaration in a matching `#ifdef`.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
typo: set the configure package version correctly.

* configure.ac (AC_INIT): The correct release number is 30.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
configury: don't use unicode characters in shell comments.

* configure.ac: Replace unicode apostrophes with ASCII equivalents.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
maint: git ignore build-aux/compile.

* build-aux/.gitignore: Add compile.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
posix: port getcwd to Hurd.

* lposix.c (Pgetcwd): Use get_current_dir_name function on Hurd.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>