add -Wno-unknown-pragmas to gcc options

this is the same as msvc's `/wd4068`

Author: ab9rf

CMakeLists.txt

@@ -224,7 +224,7 @@ if(UNIX)
     # default to hidden symbols
     # ensure compatibility with older CPUs
     add_definitions(-DLINUX_BUILD)
-    set(GCC_COMMON_FLAGS "-fvisibility=hidden -mtune=generic -Wall -Werror -Wl,--disable-new-dtags")
+    set(GCC_COMMON_FLAGS "-fvisibility=hidden -mtune=generic -Wall -Werror -Wl,--disable-new-dtags -Wno-unknown-pragmas")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${GCC_COMMON_FLAGS}")
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${GCC_COMMON_FLAGS}")
     if(DFHACK_BUILD_64)