In AC_WIRESHARK_GCC_CFLAGS_CHECK() don't check if $3 (the program to be run)

In AC_WIRESHARK_GCC_CFLAGS_CHECK() don't check if $3 (the program to be run)
exists by doing:

	if test "x$3" != "x" ; then

because if the program contains quotes it breaks the shell's parsing.
Instead test for the existence of $4 (which is mandatory if $3 is given).

Fix up the test program for -Wlogical-op so that it actually compiles
warning-free (at least on my system) when the compiler doesn't have the bug
we're checking for.

svn path=/trunk/; revision=46926