Slightly different fix for asio compilation with newer Emscripten (#1…

…2098)

This restores the previous status quo from Emscripten 3.1.28 where asio is always compiled with pthread support, however threading is still not actually functional in the Emscripten target.

The change in behaviour appears to be due to emscripten-core/emscripten@690f1bd

scripts/genie.lua

@@ -1174,7 +1174,7 @@ configuration { "asmjs" }
 		"-s EXCEPTION_CATCHING_ALLOWED=\"['_ZN15running_machine17start_all_devicesEv','_ZN12cli_frontend7executeEiPPc','_ZN8chd_file11open_commonEb','_ZN8chd_file13read_metadataEjjRNSt3__212basic_stringIcNS0_11char_traitsIcEENS0_9allocatorIcEEEE','_ZN8chd_file13read_metadataEjjRNSt3__26vectorIhNS0_9allocatorIhEEEE','_ZNK19netlist_mame_device19base_validity_checkER16validity_checker']\"",
 	}
 	defines {
-		"ASIO_DISABLE_THREADS",
+		"ASIO_HAS_PTHREADS",
 	}
 	linkoptions {
 		"-Wl,--start-group",