Use correct libraries when compiling statically (drogonframework#2136)

When compiling statically, cmake pulls shared libraries as dependencies
for drogon e.g. libpq.so instead of libpq.a. This causes linkage errors
when compiling the whole program.

The flag USE_STATIC_LIBS_ONLY should set the CMAKE_FIND_LIBRARY_SUFFIXES
to the appropriate suffix on different platforms, thus find_* commands
find the right library.

CMakeLists.txt

@@ -13,6 +13,7 @@ option(BUILD_DOC "Build Doxygen documentation" OFF)
 option(BUILD_BROTLI "Build Brotli" ON)
 option(BUILD_YAML_CONFIG "Build yaml config" ON)
 option(USE_SUBMODULE "Use trantor as a submodule" ON)
+option(USE_STATIC_LIBS_ONLY "Use only static libraries as dependencies" OFF)
 
 include(CMakeDependentOption)
 CMAKE_DEPENDENT_OPTION(BUILD_POSTGRESQL "Build with postgresql support" ON "BUILD_ORM" OFF)
@@ -77,6 +78,10 @@ if (BUILD_SHARED_LIBS)
     endif ()
 endif (BUILD_SHARED_LIBS)
 
+if(USE_STATIC_LIBS_ONLY)
+    set(CMAKE_FIND_LIBRARY_SUFFIXES "${CMAKE_STATIC_LIBRARY_SUFFIX}")
+endif(USE_STATIC_LIBS_ONLY)
+
 if(USE_SPDLOG)
   find_package(spdlog CONFIG)
   if(spdlog_FOUND)