header-vars: simplify how make flags are determined

header-vars: simplify how make flags are determined

Actually, son far only the '-n' option ("dry mode") was detected,
but this change will allow us to soon detect more options.

* lib/am/header-vars.am (am__running_with_option): Even when $MAKEFLAGS
appears to contain definition of variables with embedded whitespace,
use simple textual pre-processing over $MAKEFLAGS rather than tricky
recursive invocations of make to determine whether the '-n' option was
given.  This is enough to correctly handle all the tricky usages covered
in the testsuite.
* t/nodep.sh: Adjust to avoid a spurious failure.

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