Running a shell command

[Michael Talbot-Wilson]

I'd like to do something like

AC_PATH_PROG([PGCONFIG],[pg_config]...

followed by employing the path PGCONFIG in a shell command and
assigning the value returned.  If it was found, i.e. if there is a
pg_config on the system.  Within configure.ac ...

PGLIBDIR=$($PGCONFIG --libdir)

or something, then

AC_SUBST([PGLIBDIR])

But that attempt substitutes a blank.  Others like it no better.

I thought it was possible to execute shell commands in configure.ac
but a few hours searching the manual has got me nowhere.

Is such a thing possible?  How?