Trac #29319: pari spkg-configure.m4 should check for gphelp

The `gphelp` executable is needed by `cypari2` at installation time.

It is provided by the separate package `pari-doc` on Debian.

This causes build errors on `ubuntu-eoan-standard`
(https://github.com/mkoeppe/sage/runs/502310821?check_suite_focus=true)

URL: https://trac.sagemath.org/29319
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

build/pkgs/pari/distros/debian.txt

@@ -1,3 +1,5 @@
 pari-gp2c libpari-dev
+# #29319: cypari2 needs gphelp at installation time
+pari-doc
 # We add these data packages because they are checked by spkg-configure.m4
 pari-elldata pari-galdata pari-galpol pari-seadata

build/pkgs/pari/spkg-configure.m4

@@ -15,6 +15,13 @@ SAGE_SPKG_CONFIGURE([pari], [
         AC_MSG_NOTICE([gp is not found])
         sage_spkg_install_pari=yes
     else
+        AC_PATH_PROG([GPHELP], [gphelp]) dnl needed for cypari2 installation; see #29319
+        if test x$GPHELP = x; then
+            AC_MSG_NOTICE([gphelp is not found; cannot use system pari/GP without gphelp])
+            AC_MSG_NOTICE([Install a system package that provides it, possibly pari-doc.])
+            AC_MSG_NOTICE([Otherwise Sage will build its own pari/GP.])
+            sage_spkg_install_pari=yes
+        fi
         AC_MSG_CHECKING([is pari_elldata installed? ])
         gp_ell_check=`echo "r=ellinit(\"11a1\"); r[[11]]" | $GP -qf 2>> config.log`
         if test x$gp_ell_check = x20008; then