tango.pc does not work when installing with a CMAKE_INSTALL_LIBDIR that is not relative to the CMAKE_INSTALL_PREFIX

For example, configuring with the following on main (baa92128 (Merge branch 'add-version-macros' into 'main', 2025-11-13):

cmake -Bbuild -S. -DTANGO_WARNINGS_AS_ERRORS=OFF -DCMAKE_INSTALL_PREFIX=/opt/tri/test-prefix -DCMAKE_INSTALL_LIBDIR=/opt/tri/test-prefix/lib64

Produces the following tango.pc file on my system:

> cat /opt/tri/test-prefix/lib64/pkgconfig/tango.pc 
# SPDX-FileCopyrightText: 2016 Copyright contributors to the cppTango project
#
# SPDX-License-Identifier: LGPL-3.0-or-later

prefix=/opt/tri/test-prefix
exec_prefix=${prefix}/bin
includedir=${prefix}/include
libdir=${prefix}//opt/tri/test-prefix/lib64

Name: cppTango
Description: Tango client/server API library
Version: 10.3.0dev
Cflags: -I${includedir}
Requires: libzmq omniORB4 omniCOS4 omniDynamic4
Requires.private: libjpeg
Libs: -L${libdir} -ltango -lzmq -lomniORB4 -lomnithread -lCOS4 -lomniDynamic4
Libs.private: -ljpeg