Broken pkg-config file ompl.pc

OMPL generates a broken pkg-config file since PR #1066

(Following outputs are generated inside an Ubuntu 20.04 docker container, with -DCMAKE_INSTALL_PREFIX=/opt/workspace/install)

Before #1066 (good):

# This file was generated by CMake for ompl
prefix=/opt/workspace/install
exec_prefix=${prefix}
libdir=${prefix}/lib
includedir=${prefix}/include/ompl-1.6

Name: ompl
Description: The Open Motion Planning Library
Version: 1.6.0
Requires: eigen3 
Libs: -L${libdir} -lompl /usr/lib/x86_64-linux-gnu/libboost_serialization.so /usr/lib/x86_64-linux-gnu/libboost_filesystem.so /usr/lib/x86_64-linux-gnu/libboost_system.so -lpthread
Cflags: -std=c++17 -I${includedir} -I/usr/include

After #1066 (bad, Boost::* are no valid pkg-config dependencies):

# This file was generated by CMake for ompl
prefix=/opt/workspace/install
exec_prefix=${prefix}
libdir=${prefix}/lib
includedir=${prefix}/include/ompl-1.6

Name: ompl
Description: The Open Motion Planning Library
Version: 1.6.0
Requires: eigen3 Boost::filesystem Boost::program_options Boost::serialization Boost::system
Libs: -L${libdir} -lompl -lpthread
Cflags: -std=c++17 -I${includedir} -I/usr/include

Trying to use pkg-config will thus lead to an error:

$ pkg-config ompl --cflags
Package Boost::filesystem was not found in the pkg-config search path.
Perhaps you should add the directory containing `Boost::filesystem.pc'
to the PKG_CONFIG_PATH environment variable
Package 'Boost::filesystem', required by 'ompl', not found

On the current main branch the following ompl.pc is generated (which is even more broken):

# This file was generated by CMake for ompl
prefix=/opt/workspace/install
exec_prefix=${prefix}
libdir=/opt/workspace/install/lib
includedir=/opt/workspace/install/include

Name: ompl
Description: The Open Motion Planning Library
Version: 1.6.0
Requires: Eigen3::Eigen Boost::filesystem Boost::serialization Boost::system Eigen3::Eigen $<$<BOOL:TRUE>:Threads::Threads> $<$<BOOL:>:flann::flann> $<$<BOOL:FALSE>:Triangle::Triangle> $<$<BOOL:>:Spot::Spot>
Libs: -L${libdir} -lompl 
Cflags: -std=c++17 -I${includedir} -I/usr/include

Start by inspecting the CMake/pkg-config generation changed by PR #1066 and comparing the generated ompl.pc examples in this report. Reproduce the install in an Ubuntu 20.04 container with -DCMAKE_INSTALL_PREFIX=/opt/workspace/install, then verify pkg-config ompl --cflags and --libs. Done means ompl.pc contains valid pkg-config dependencies and works without treating CMake target names as package names.