Incorrect reference to boost thread library in pkg-config file on macOS

[scpeters]

Copied from #2751 (review):

I've noticed for a while that the pkg-config files have incorrect references to the boost libraries on homebrew, passing cmake target names to the linking args like -lBoost::thread instead of -lboost_thread. #2751 does some string manipulation to fix this, but it doesn't fix the issue on homebrew because those libraries have an extra -mt (for multithreaded I believe).

$ ls /usr/local/opt/boost/lib | grep thread
libboost_thread-mt.a
libboost_thread-mt.dylib

This causes CI error messages like the following when trying to compile example code based on pkg-config flags:

399: ld: library not found for -lboost_thread

In my previous debugging, I tried to use a generator expression to extract the boost library names from cmake targets. For example, the following patch is able to write the absolute path of the library for each target:

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8659ff1707..e90e868a23 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -383,6 +383,7 @@ else (build_errors)
   string (REPLACE ";" " -L" Boost_PKGCONFIG_LIBS "${Boost_PKGCONFIG_LIBS}")
 
   foreach (b ${Boost_LIBRARIES})
+    file(GENERATE OUTPUT generated_${b}.txt CONTENT $<TARGET_LINKER_FILE:${b}>\n)
     get_filename_component(bname ${b} NAME_WE)
     # Prefix -l if not already prefixed
     string(FIND ${bname} "-l" lprefix)

$ cat generated_Boost\:\:thread.txt 
/usr/local/lib/libboost_thread-mt.dylib

cmake has a different generator expression that seems like it would be perfect for this case: TARGET_LINKER_FILE_BASE_NAME, but it doesn't work for boost and has been reported upstream (boostorg/boost_install#41, and cmake issue 20611).