Added patch for Emscripten 1.39.16 (closes #16)

Thanks @reneeb!

build/build.pl

@@ -97,6 +97,8 @@ my $needs_reconfig = !!$opts{reconfig};
 	my $patchf;
 	if ( $libraryjs=~/\b\Q___setErrNo(ERRNO_CODES.\E(EAGAIN|ENOTSUP)\b/ )
 		{ $patchf = 'emscripten_1.38.10_eagain.patch' }
+	elsif ( $libraryjs=~/no shell available\s+setErrNo\Q({{{ cDefine('EAGAIN') }}})\E/ )
+		{ $patchf = 'emscripten_1.39.16_eagain.patch' }
 	elsif ( $libraryjs=~/\b\QcDefine('EAGAIN')\E/ ) # note that this appears in 1.38.1* versions too
 		{ $patchf = 'emscripten_1.38.28_eagain.patch' }
 	else { die "Could not figure out which library.js patch to use" }

build/emscripten_1.39.16_eagain.patch

@@ -0,0 +1,19 @@
+--- library.js.orig     2020-05-18 17:14:18.682328912 +0200
++++ library.js  2020-05-18 17:14:48.366639562 +0200
+@@ -271,7 +271,7 @@
+     // pid_t fork(void);
+     // http://pubs.opengroup.org/onlinepubs/000095399/functions/fork.html
+     // We don't support multiple processes.
+-    setErrNo({{{ cDefine('EAGAIN') }}});
++    setErrNo({{{ cDefine('ENOTSUP') }}});
+     return -1;
+   },
+   vfork: 'fork',
+@@ -696,7 +696,7 @@
+     // http://pubs.opengroup.org/onlinepubs/000095399/functions/system.html
+     // Can't call external programs.
+     if (!command) return 0; // no shell available
+-    setErrNo({{{ cDefine('EAGAIN') }}});
++    setErrNo({{{ cDefine('ENOTSUP') }}});
+     return -1;
+   },