emsciptenStdenv: allow reuse without ./autogen.sh

The configure phase of emscriptenStdenv exspects an ./autogen.sh script,
which is anoying when this step is not needed. Good example is
emscriptenPackages.zlib which needs to override configurePhase.

pkgs/development/em-modules/generic/default.nix

···
     HOME=$TMPDIR
     runHook preConfigure
 
-    # probably requires autotools as dependency
-    ./autogen.sh
     emconfigure ./configure --prefix=$out
 
     runHook postConfigure