Add Alias library for Drogon with name matching installed target (#2456)

- When using the library as a downloaded package, either by using as a
  submodule or downloading the source, we can only link to it by using
  the project name which is "drogon"
- We can add an Alias library with the same target name that is used
  when the project is installed.
- This way the cmake "target_link_library" Command would always refer to the
  library by the same name which is "Drogon::Drogon" while also avoiding
  the pitfall of the default behaviour of the command just adding the the target name to the
  link arguments if the library isn't found.

Co-authored-by: Mathew Benson <mathew@benson.co.ke>

Author: MathewBensonCode

CMakeLists.txt

@@ -134,6 +134,8 @@ if (WIN32)
         PRIVATE $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/third_party/mman-win32>)
 endif (WIN32)
 
+add_library(Drogon::Drogon ALIAS ${PROJECT_NAME})
+
 if(USE_SUBMODULE)
 add_subdirectory(trantor)
 target_link_libraries(${PROJECT_NAME} PUBLIC trantor)