Make draco.pc usable: emit absolute paths.

cmake/draco_install.cmake fills the template from the *relative* GNUInstallDirs
values (includes_path = "include", libs_path = "lib64"), so the installed
draco.pc reads

    Cflags: -Iinclude
    Libs: -Llib64 -ldraco

Those are relative to whatever directory the consumer happens to compile in, so
`pkg-config --cflags --libs draco` hands the toolchain two paths that do not
exist and the library is undiscoverable by pkg-config.  GNUInstallDirs also
defines CMAKE_INSTALL_FULL_INCLUDEDIR / CMAKE_INSTALL_FULL_LIBDIR, which are the
same directories anchored at CMAKE_INSTALL_PREFIX; use those instead, and expose
them through the conventional prefix/libdir/includedir variables so the file can
be relocated the way every other .pc can.

Not filed upstream: 1.5.7 (2024-01-17) is still the newest release and this
overlay does not submit patches upstream.

--- a/cmake/draco.pc.template
+++ b/cmake/draco.pc.template
@@ -1,6 +1,11 @@
+prefix=@CMAKE_INSTALL_PREFIX@
+exec_prefix=${prefix}
+libdir=@CMAKE_INSTALL_FULL_LIBDIR@
+includedir=@CMAKE_INSTALL_FULL_INCLUDEDIR@
+
 Name: @PROJECT_NAME@
 Description: Draco geometry de(com)pression library.
 Version: @DRACO_VERSION@
-Cflags: -I@includes_path@
-Libs: -L@libs_path@ -ldraco
+Cflags: -I${includedir}
+Libs: -L${libdir} -ldraco
 Libs.private: @CMAKE_THREAD_LIBS_INIT@