Build a third version of libpgcommon.a, with -fPIC and -DFRONTEND,
as commit ea53100d did for src/port.  Use that in libpq to avoid
symlinking+rebuilding source files retail.

Also adjust ecpg to use the new src/port and src/common libraries.

Arrange to install these libraries, too, to simplify out-of-tree
builds of shared libraries that need any of these modules.

Discussion: https://postgr.es/m/13022.1538003440@sss.pgh.pa.us
Discussion: https://postgr.es/m/E1g5Y8r-0006vs-QA@gemulon.postgresql.org