spkg-configure for pari_elldata

if pari/GP is available, but elldata is not, then we
need to vendor pari/GP, as we cannot (easily) just install elldata

build/pkgs/pari_elldata/spkg-configure.m4

@@ -0,0 +1,21 @@
+SAGE_SPKG_CONFIGURE([pari_elldata], [
+    AC_REQUIRE([SAGE_SPKG_CONFIGURE_PARI])
+    AC_MSG_CHECKING([installing pari? ])
+    if test x$sage_spkg_install_pari = xyes; then
+        AC_MSG_RESULT([yes; install pari_elldata as well])
+        sage_spkg_install_pari_elldata=yes
+    else
+        AC_MSG_RESULT([no])
+        AC_MSG_CHECKING([is pari_elldata installed? ])
+        gp_ell_check=`echo "r=ellinit(\"11a1\"); r[[11]]" | $GP -qf`
+        if test x$gp_ell_check = x20008; then
+            AC_MSG_RESULT([yes])
+        else
+            AC_MSG_RESULT([no; cannot use system pari/GP without elldata package])
+            AC_MSG_NOTICE([Install elldata package and reconfigure.])
+            AC_MSG_NOTICE([Otherwise Sage will build its own pari/GP.])
+            sage_spkg_install_pari_elldata=yes
+            sage_spkg_install_pari=yes
+        fi 
+    fi
+])