Fix ./configure tests (#1151)

Fix ./configure tests (#1151)

Many tests broken by recent commit a1c2236 supplying empty input to
AC_LINK_IFELSE() macro in SQUID_CC_CHECK_ARGUMENT. Empty input results
in no test file created by the macro (and, presumably, a failed linking
test). Observable symptoms (in some environments) include many repeated
errors on ./configure stderr:

    sed: can't read conftest.cpp: No such file or directory

Also fixed a similar SQUID_SEARCH_LIBS() bug. That macro was broken
since inception (commit 391f0ba). It is currently only used for the
mingw-specific SQUID_CHECK_WINSOCK_LIB check. This change is untested.