Use pkg-config's PKG_CHECK_MODULES to check for and link with libecap.

Use pkg-config's PKG_CHECK_MODULES to check for and link with libecap.

Side-effect: We can and do check whether Squid supports the installed libecap
package version. This check avoids accidently building Squid with the wrong
libecap release. TODO: Check whether the loaded eCAP adapter was build with a
supported libecap version as well.