cmake: install pkg-config file relative to current_binary_dir

When the testsuite is enabled, the "cmake_add_subdirectory" test
adds a second copy of the project into the build configuration, and the
project files are installed twice.

This becomes super problematic when it tries to install a file from
CMAKE_BINARY_DIR which is only available in CMAKE_CURRENT_BINARY_DIR
and bombs out with the following error message:

```
[...]
-- Installing: <DESTDIR>/usr/lib/cmake/nlohmann_json/nlohmann_jsonTargets.cmake
CMake Error at test/cmake_add_subdirectory/nlohmann_json/cmake_install.cmake:73 (file):
  file INSTALL cannot find
  "......./nlohmann-json/builddir/test/cmake_add_subdirectory/nlohmann_json.pc":
  No such file or directory.
Call Stack (most recent call first):
  test/cmake_add_subdirectory/cmake_install.cmake:47 (include)
  test/cmake_install.cmake:49 (include)
  cmake_install.cmake:94 (include)
```

CMakeLists.txt

@@ -157,7 +157,7 @@ endif()
         DESTINATION ${NLOHMANN_JSON_CONFIG_INSTALL_DIR}
     )
     install(
-        FILES "${CMAKE_BINARY_DIR}/${PROJECT_NAME}.pc"
+        FILES "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"
         DESTINATION lib/pkgconfig
     )
 endif()