Add -fno-gnu-unique to compile options for gcc

Unique symbols prevent the library from being dlclosed.

Author: mateusztabaka

CMakeLists.txt

@@ -118,6 +118,9 @@ if(NOT MSVC)
     else()
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fpermissive")
     endif()
+    if(CMAKE_CXX_COMPILER_ID STREQUAL GNU)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-gnu-unique")
+    endif()
     if(NOT CMAKE_CXX_COMPILER_ID STREQUAL IntelLLVM)
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wnon-virtual-dtor")
     endif()