remake: behave better with non-GNU make in subdirectories

remake: behave better with non-GNU make in subdirectories

Currently, with every decent make program, it is possible to
rebuild out-of-date autotools-generated files with a simple
"make Makefile" -- but for this to work reliably with non-GNU
make implementations, the command must be issued from the
top-level directory.  This patch removes such limitation.

* lib/am/configure.am (am--refresh): Depend on `%MAKEFILE%'.
* tests/defs.in (using_gmake): New function, backported from the
`master' branch (and simplified).
* tests/remake-subdir.test: New test.
* tests/remake-subdir2.test: Likewise.
* tests/remake-subdir-gnu.test: Likewise.
* tests/remake-subdir-from-subdir.test: Likewise.
* tests/Makefile.am (TESTS): Update.