build: add Libs/Cflags to libzpc.pc and fix libdir substitution

[plusky]

libzpc.pc is installed with no Libs: and no Cflags:, so pkg-config --libs libzpc returns nothing for a package that does ship a linkable shared library. A consumer built with gcc $(pkg-config --cflags --libs libzpc) prog.c gets no library on the link line and fails with undefined references to zpc_*. libdir and includedir are declared in the template and then never referenced by any field.

This is reachable in practice: SUSE Linux Enterprise ships libzpc-devel from the 1.x line (1.3.0 on SLE 15 SP7, 1.5.0 on SLE 16.0) and the shipped .pc has this shape. Tracked downstream as https://bugzilla.suse.com/show_bug.cgi?id=1279335.

Two hunks, because the one-liner alone would be wrong:

1. libzpc.pc.in gains Libs: -L${libdir} -lzpc and Cflags: -I${includedir}.
2. include(GNUInstallDirs) moves above configure_file(libzpc.pc.in ...). It currently runs after, so @CMAKE_INSTALL_LIBDIR@ and @CMAKE_INSTALL_INCLUDEDIR@ expand to nothing unless the caller passes -DCMAKE_INSTALL_LIBDIR / -DCMAKE_INSTALL_INCLUDEDIR. Distro builds usually do, which is why the shipped libdir looks correct, but a plain cmake -DCMAKE_INSTALL_PREFIX=/usr does not — and then the new Libs: line would point at ${exec_prefix}/.

Before, plain cmake -DCMAKE_INSTALL_PREFIX=/usr:

libdir=${exec_prefix}/
includedir=${prefix}/

Name: libzpc
Description: IBM Z Protected-key Crypto library
Version: 1.5.0

After, same invocation:

libdir=${exec_prefix}/lib64
includedir=${prefix}/include

Name: libzpc
Description: IBM Z Protected-key Crypto library
Version: 1.5.0
Libs: -L${libdir} -lzpc
Cflags: -I${includedir}

$ pkg-config --cflags --libs libzpc
 -lzpc

(-L/usr/lib64 and -I/usr/include are correctly elided by pkg-config as default search paths.)

Targeted at libzpc-1.5.y rather than main: on main, e91faa1 removed the shared-library, header and pkg-config installs, so libzpc.pc.in is no longer used there and this fix would be a no-op.

No API or ABI change. No regression test added — the project's test suite is a gtest binary for the library API and has no hook for checking generated build metadata; happy to add one if you'd like it somewhere.