ESI: Replace --disable-esi build option

[yadij]

What this build option did can also be performed using both options "--without-expat --without-xml2".

Check libexpat and libxml2 with SQUID_CHECK_LIB_WORKS. Expand the macro to auto-define make variable ENABLE_LIBFOO and compiler variable HAVE_LIBFOO for working libraries.

Existing libraries checked with this macro do not need those variables, but many others yet to be converted will.

Also, require pkg-check for library detection. Removing the manual check for 'main()' function in the library ABI.