dry-run: don't get confused by '-I' option

Fixes automake bug#13760 for non-GNU make implementations that still
support the option '-I'.  So far, the only such make implementation
are FreeBSD (8.x) make and NetBSD (5.x) make.

* lib/am/header-vars.am (am__make_dryrun): If a non-GNU make is being
used, try to handle the '-I' option in $MAKEFLAGS correctly.  For GNU
make, that is already done by the proper use of the $MFLAGS variable.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>