Fix pkg-config boost entries for Ubuntu Focal (gazebo9)

[j-rivero]

Partial fix for #2735 . Need another PR for Gazebo11

[scpeters]

I've noticed this issue for a while on homebrew since it has had a newer boost version for a while. Unfortunately, this 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

The homebrew CI build is gone now, but I saw the following in an email about it:

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 that been reported as a known issue for these boost targets (boostorg/boost_install#41, and cmake issue 20611)

so I'll approve this because it seems to work for ubuntu 20.04, but it doesn't fix homebrew

[scpeters]

I've updated the changelog and also bumped the version to 9.13.2 in anticipation of an upcoming release

[scpeters]

I filed #2790 to capture the issues with homebrew that I discussed in that comment