Support discovery of TinyXML2 by pkgconfig

Allow to discovery TinyXML2 by pkg-config is a `-config.cmake` file is not available.

Since tinyxml2 is built with meson in Yocto it's configuration gets deployed by `pkg-config` (because meson cannot generate CMake target files). In this case, the current `FindTinyXML2.cmake` will find out the path of `tinyxml2.h` and of `tinyxml2.so` by looking into the `recipe-sysroot` folder, which path in going to get expanded into the `fastrtps-target.cmake` generated when fastdds is built. The end result is that we're going to have a system-wide deployed config mode package that points to a directory that won't (probably) not exist anymore when other dependent targets are built.

Since it's not possible to force meson to produce a `tinyxml2-target.cmake` file, the most reasonable solution is trying to look for the library by pkg-config.

With this patch applied, the generated `fastrtps-target.cmake` will have a reference to the target `PkgConfig::TinyXML2` instead of a full path.

Author: Federico Giovanardi

cmake/modules/FindTinyXML2.cmake

@@ -15,6 +15,13 @@ endif()
 
 if(NOT (TINYXML2_FROM_SOURCE OR TINYXML2_FROM_THIRDPARTY))
     find_package(TinyXML2 CONFIG QUIET)
+    if(NOT TinyXML2_FOUND)
+        find_package(PkgConfig REQUIRED)
+        pkg_check_modules(TinyXML2 REQUIRED IMPORTED_TARGET tinyxml2)
+        if(TARGET PkgConfig::TinyXML2)
+            add_library(tinyxml2::tinyxml2 ALIAS PkgConfig::TinyXML2)
+        endif()
+    endif()
 endif()
 
 if(TinyXML2_FOUND AND NOT TINYXML2_FROM_THIRDPARTY)

cmake/packaging/Config.cmake.in

@@ -25,6 +25,13 @@ set_and_check(@PROJECT_NAME@_LIB_DIR "@PACKAGE_LIB_INSTALL_DIR@")
 find_package(fastcdr REQUIRED)
 find_package(foonathan_memory REQUIRED)
 find_package(TinyXML2 QUIET)
+if(NOT TinyXML2_FOUND)
+    find_package(PkgConfig REQUIRED)
+    pkg_check_modules(TinyXML2 IMPORTED_TARGET tinyxml2)
+    if(TARGET PkgConfig::TinyXML2)
+        add_library(tinyxml2::tinyxml2 ALIAS PkgConfig::TinyXML2)
+    endif()
+endif()
 @FASTDDS_INSTALLER_DEPS_ANCILLARY@
 @FASTDDS_PACKAGE_UNIX_OPT_DEPS@