Fix the SQUID_CC_REQUIRE_ARGUMENT autoconf function (#478)

Fix the SQUID_CC_REQUIRE_ARGUMENT autoconf function (#478)

Inside AC_DEFUN(), autoconf replaces `$1` with the first argument of the
function.  In this case, the first argument is a variable name. To get
the _value_ of that variable, one has to use `$$1`.

One known effect of this fix (in many build environments) is the
disappearance of the following annoying extra error when a build fails
for some other reason:

    unrecognized command line option -Wno-deprecated-register