build/cmake: export used libraries to .pc file

CMakeLists.txt

@@ -63,6 +63,7 @@ if (NOT MSVC)
         find_package (Readline 6 REQUIRED)
     endif ()
     pkg_search_module(argon2 REQUIRED libargon2)
+    set(argon2_lib ", libargon2")
     pkg_search_module(Jsoncpp jsoncpp)
     if (Jsoncpp_FOUND)
         add_definitions(-DOPENDHT_JSONCPP)
@@ -82,9 +83,11 @@ if (NOT MSVC)
         add_library(fmt SHARED IMPORTED)
         find_library(HTTP_PARSER_LIBRARY http_parser)
         add_library(http_parser SHARED IMPORTED)
+        set(http_parser_lib "-lhttp_parser ")
         if (NOT Jsoncpp_FOUND)
             message(SEND_ERROR "Jsoncpp is required for DHT proxy support")
         endif()
+        set(jsoncpp_lib ", jsoncpp")
         if (OPENDHT_PROXY_OPENSSL)
             # https://cmake.org/cmake/help/latest/module/FindOpenSSL.html
             pkg_search_module(OPENSSL REQUIRED openssl)
@@ -95,6 +98,7 @@ if (NOT MSVC)
             else ()
                 message(SEND_ERROR "OpenSSL is required for DHT proxy as specified")
             endif()
+            set(openssl_lib ", openssl")
         endif()
     else ()
         set(OPENDHT_PROXY_OPENSSL OFF)