Add option to drop privileges with pledge().
From https://github.com/mgba-emu/mgba/pull/1271.

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -47,6 +47,7 @@ if(NOT LIBMGBA_ONLY)
 		set(USE_EDITLINE ON CACHE BOOL "Whether or not to enable the CLI-mode debugger")
 	endif()
 	set(USE_GDB_STUB ON CACHE BOOL "Whether or not to enable the GDB stub ARM debugger")
+	set(USE_PLEDGE_UNVEIL OFF CACHE BOOL "Whether or not to drop privileges with pledge and unveil")
 	set(USE_FFMPEG ON CACHE BOOL "Whether or not to enable FFmpeg support")
 	set(USE_ZLIB ON CACHE BOOL "Whether or not to enable zlib support")
 	set(USE_MINIZIP ON CACHE BOOL "Whether or not to enable external minizip support")
@@ -540,6 +541,14 @@ if(USE_GDB_STUB)
 endif()
 source_group("Debugger" FILES ${DEBUGGER_SRC})
 
+if(USE_PLEDGE_UNVEIL)
+	set(USE_EPOXY OFF)
+endif()
+
+if(USE_PLEDGE_UNVEIL)
+	list(APPEND FEATURES PLEDGE_UNVEIL)
+endif()
+
 if(USE_FFMPEG)
 	list(APPEND FEATURES FFMPEG)
 	if(LIBSWRESAMPLE_FOUND)
@@ -1281,6 +1290,7 @@ if(NOT QUIET AND NOT LIBMGBA_ONLY)
 		message(STATUS "	CLI debugger: ${USE_EDITLINE}")
 	endif()
 	message(STATUS "	GDB stub: ${USE_GDB_STUB}")
+	message(STATUS "        pledge/unveil: ${USE_PLEDGE_UNVEIL}")
 	message(STATUS "	GIF/Video recording: ${USE_FFMPEG}")
 	message(STATUS "	Screenshot/advanced savestate support: ${USE_PNG}")
 	message(STATUS "	ZIP support: ${SUMMARY_ZIP}")