Merge branch '958-improve-message-about-python-ply' into 'master'

Improve the error message about missing PLY Python package

Closes #958

See merge request isc-projects/bind9!1918

configure

@@ -12586,7 +12586,7 @@ fi
 $as_echo_n "checking for Python support... " >&6; }
 	      { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
 $as_echo "no" >&6; }
-	      as_fn_error $? "Python required for dnssec-keymgr" "$LINENO" 5
+	      as_fn_error $? "Python >= 2.7 or >= 3.2 and the PLY package are required for dnssec-keymgr and other Python-based tools. PLY may be available from your OS package manager as python-ply or python3-ply; it can also be installed via pip. To build without Python/PLY, use --disable-python." "$LINENO" 5
 fi
 fi
 

configure.ac

@@ -262,7 +262,15 @@ AS_IF([test "$with_python" = "no"],
        AS_IF([test "X$PYTHON" = "X"],
 	     [AC_MSG_CHECKING([for Python support])
 	      AC_MSG_RESULT([no])
-	      AC_MSG_ERROR([Python required for dnssec-keymgr])])])
+	      AC_MSG_ERROR([m4_normalize(
+				[Python >= 2.7 or >= 3.2 and the PLY package
+                                 are required for dnssec-keymgr and other
+                                 Python-based tools. PLY may be
+                                 available from your OS package manager
+                                 as python-ply or python3-ply; it can also
+                                 be installed via pip. To build without
+                                 Python/PLY, use --disable-python.]
+			    )])])])
 
 PYTHON_TOOLS=''
 CHECKDS=''