Added -pthread to gcc

At least gcc-5.4 on Ubuntu 16.04 needs it. It shouldn't hurt on older versions.

CMakeLists.txt

@@ -25,7 +25,7 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
 			set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")
 		endif()
 	endif()
-	# Note: gcc is fine without -pthreads.
+	set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -pthread")
 elseif("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC") # the quotes are needed here, maybe because "MSVC" seems to be a keyword
 	if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19)
 		message(FATAL_ERROR "Visual Studio 2015 or newer is required.")