Streamline ./configure handling of optional libraries (#606)

Streamline ./configure handling of optional libraries (#606)

Squid macros to simplify most AC_ARG_WITH behaviour.

These macros perform the necessary checks for on/off and
custom library path validation for Squid coding policy.

Usage:
* When the parameter is for a build value rather than a
  library to link. Use AC_ARG_WITH.

* When a library is experimental and should default disable.
   Use SQUID_OPTIONAL_LIB to detect the --with parameter.

* When a library is expected to be available and default enable.
  Use SQUID_AUTO_LIB to detect --without parameter.

Sets pkg-config variables to enable more consistent configure.ac
logic for libraries even if pkg-config is not used or supported
for any specific library.

Fixes several trivial bugs where unnecessary or duplicate
library entries were injected into CXXFLAGS, LDFLAGS and similar.
Followup logic for some libraries is adjusted to inject the
pkg-config variables to retain build behaviour for now.