fix problem with building in the source directory

The symptom was this error message:

    checking for gtest... checking for /Users/dan/src/M2/M2-Macaulay2/M2/usr-host/include/gtest/src/gtest-all.cc... no
    configure: error: could not find gtest source at path /Users/dan/src/M2/M2-Macaulay2/M2/usr-host/include/gtest.
    GNUmakefile:119: recipe for target 'submodules/memtailor/Makefile' failed
    make: *** [submodules/memtailor/Makefile] Error 1

The problem was confusion in GNUmakefile.in, where "configure" was being used as a target to refer both to
the file "./configure", which has to be made from "configure.ac", as well as to a target signifying that
the script "./configure" should be run.
Merge branch 'master' of github.com:Macaulay2/M2