Enable multithreading on Linux by default.

src/config.h

@@ -110,6 +110,7 @@
 #	define BGFX_CONFIG_MULTITHREADED ( (!BGFX_CONFIG_RENDERER_NULL)&(0 \
 						| BX_PLATFORM_ANDROID \
 						| BX_PLATFORM_IOS \
+						| BX_PLATFORM_LINUX \
 						| BX_PLATFORM_NACL \
 						| BX_PLATFORM_OSX \
 						| BX_PLATFORM_QNX \