[SV 62088] Avoid re-exec due to stdin.

[SV 62088] Avoid re-exec due to stdin.

The fix for SV 60595 causes make to re-execute when the makefile is
read from stdin.  E.g.
    $ printf 'all:; $(info hello)' | make -sf -

Reported by Sergei Trofimovich <siarheit@google.com>
Test added by Paul Smith <psmith@gnu.org>

* src/main.c (main): Set the mtime of the stdin temporary file.
* tests/scripts/features/reinvoke: Add a test.