libdir config & pkg-config file are incorrect in source build

on a source build

cd /develop/
mkdir tmp
cd tmp
git clone https://github.com/Pulse-Eight/platform
cd platform
mkdir build
cd build
cmake .. -DCMAKE_INSTALL_PREFIX=/develop/tmp/P8 -DCMAKE_INSTALL_LIBDIR=lib64 -DBUILD_SHARED_LIBS=1

creates a PKG-CONFIG file

cat P8.pc
	prefix=/develop/tmp/P8
	libdir=/develop/tmp/P8//develop/tmp/platform/build/lib64
	includedir=/develop/tmp/P8/include
	DEPENDENCIES=-lpthread

	Name: P8
	Description: Pulse-Eight platform support library 2.1
	Version: 2.1.0
	Libs: -L${libdir} -lP8
	Cflags: -I${includedir}

where the value for libdir

	libdir=/develop/tmp/P8//develop/tmp/platform/build/lib64

is wrong. it should be just

	libdir=/develop/tmp/P8/lib64

and then

make
make install

never installs the .pc file , or the LIBS, in the target dir

make install
	[100%] Built target P8
	Install the project...
	-- Install configuration: ""
	-- Up-to-date: /develop/tmp/platform/build/lib64/libP8.so.2.1.0
	-- Up-to-date: /develop/tmp/platform/build/lib64/libP8.so.2
	-- Up-to-date: /develop/tmp/platform/build/lib64/libP8.so
	-- Installing: /develop/tmp/P8/include/P8/os.h
	-- Installing: /develop/tmp/P8/include/P8/posix/os-socket.h
	-- Installing: /develop/tmp/P8/include/P8/posix/os-threads.h
	-- Installing: /develop/tmp/P8/include/P8/posix/os-types.h
	-- Installing: /develop/tmp/P8/include/P8/sockets/cdevsocket.h
	-- Installing: /develop/tmp/P8/include/P8/sockets/socket.h
	-- Installing: /develop/tmp/P8/include/P8/sockets/tcp.h
	-- Installing: /develop/tmp/P8/include/P8/threads/atomics.h
	-- Installing: /develop/tmp/P8/include/P8/threads/mutex.h
	-- Installing: /develop/tmp/P8/include/P8/threads/threads.h
	-- Installing: /develop/tmp/P8/include/P8/util/atomic.h
	-- Installing: /develop/tmp/P8/include/P8/util/buffer.h
	-- Installing: /develop/tmp/P8/include/P8/util/StringUtils.h
	-- Installing: /develop/tmp/P8/include/P8/util/StdString.h
	-- Installing: /develop/tmp/P8/include/P8/util/timeutils.h
	-- Installing: /develop/tmp/P8/include/P8/util/util.h
	-- Up-to-date: /develop/tmp/platform/build/lib64/pkgconfig/P8.pc
	-- Up-to-date: /develop/tmp/platform/build/lib64/P8/P8-config.cmake

	find /develop/tmp/P8 | grep pc
		empty result
	```