build: parametrize Python invocations in the shebangs

Consequently, there's no reason to have the affected files marked as
executable (processing the files by the means of AC_CONFIG_FILES will
get rid of any such permission bits, anyway), but at the very least,
test/runtests.py needs to be set executable afterwards so as no to
cause failures with the current "make check" arrangement that invokes
TESTS subtargets directly (i.e. no extension-based LOG_COMPILER set).

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>