Add missing library target .pc files

kj-test and kj-http currently have their headers installed, and the CMake
build exports them as targets, so I'm adding .pc.in files for them. I
initially added capnpc as well, but that appears to be a private API.

c++/kj-http.pc.in

+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: KJ HTTP Library
+Description: Basic utility library called KJ (HTTP part)
+Version: @VERSION@
+Libs: -L${libdir} -lkj-http @PTHREAD_CFLAGS@ @PTHREAD_LIBS@ @STDLIB_FLAG@
+Requires: kj-async = @VERSION@
+Cflags: -I${includedir} @PTHREAD_CFLAGS@ @STDLIB_FLAG@ @CAPNP_LITE_FLAG@

c++/kj-test.pc.in

+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: KJ Test Framework
+Description: Basic utility library called KJ (test part)
+Version: @VERSION@
+Libs: -L${libdir} -lkj-test @PTHREAD_CFLAGS@ @PTHREAD_LIBS@ @STDLIB_FLAG@
+Requires: kj = @VERSION@
+Cflags: -I${includedir} @PTHREAD_CFLAGS@ @STDLIB_FLAG@ @CAPNP_LITE_FLAG@