Add -flto as link flag, also fix missing <string> include in main.cpp causing a build failure with gcc10

CMakeLists.txt

@@ -22,6 +22,7 @@ endif()
 
 if(ENABLE_LTO)
 	add_compile_options(-O3 -flto)
+	add_link_options(-flto)
 	set(CMAKE_AR "gcc-ar")
 	set(CMAKE_C_ARCHIVE_CREATE "<CMAKE_AR> qcs <TARGET> <LINK_FLAGS> <OBJECTS>")
 	set(CMAKE_C_ARCHIVE_FINISH   true)

src/libui_sdl/libui/unix/CMakeLists.txt

@@ -65,7 +65,7 @@ macro(_handle_static)
 		OUTPUT ${_oname}
 		DEPENDS ${_LIBUINAME}
 		COMMAND
-			ld -r --whole-archive ${_aname} -o ${_oname}
+			${CMAKE_LINKER} -r --whole-archive ${_aname} -o ${_oname}
 		COMMAND
 			objcopy --localize-hidden ${_oname}
 		COMMENT "Removing hidden symbols")

src/libui_sdl/main.cpp

@@ -20,6 +20,7 @@
 #include <time.h>
 #include <stdio.h>
 #include <string.h>
+#include <string>
 
 #ifndef __WIN32__
 #include <glib.h>