HTML5: Do not error on undefined symbols

Emscripten used not to error out on those warnings, but this behaviour changed
recently, so we revert it to be able to compile as we used to for previous 2.1.x
releases. It would be good to fix those undefined symbols though.

platform/javascript/detect.py

@@ -96,6 +96,12 @@ def configure(env):
     env.Append(LINKFLAGS=['-O2'])
     # env.Append(LINKFLAGS=['-g4'])
 
+    # FIXME: This used to be the default in older emscripten, but now it would error out.
+    # We have two undefined symbols in the current 2.1 branch (since years):
+    # error: undefined symbol: popen
+    # error: undefined symbol: sem_getvalue
+    env.Append(LINKFLAGS=['-s', 'ERROR_ON_UNDEFINED_SYMBOLS=0'])
+
     # print "CCCOM is:", env.subst('$CCCOM')
     # print "P: ", env['p'], " Platofrm: ", env['platform']