separate debug info file for gdb

in M2/Macaulay2/bin/Makefile.in we sometimes put the debug info into a separate file. If we're going to take that seriously, we have to teach gdb how to find it, as explained in https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html

Start with M2/Macaulay2/bin/Makefile.in and read the linked GDB documentation on separate debug files. Trace how debug information is currently separated during the build and verify the resulting executable lets GDB locate it automatically. Done means the build's separate debug information is discoverable by GDB.

Written by the indexing model from the issue text.