Improve glgen

+ gen script is really a bash script rather than a sh script,
  so declare that to be true. (For example, it uses pushd,
  which is a part of bash, but not a part of sh. Not sure
  how this worked until now. Possibly gen was only run in
  environments where /bin/sh was really bash.

+ Check the results of the java compile of the code generator,
  and abort the script if the compile fails.

+ Turn on the bash shell option that guards against using
  uninitialized variables in the script.

+ Remove the generated class files.

Refactor JniCodeEmitter into two classes: a general-purpose
JniCodeEmitter and a specific Jsr239CodeEmitter. The hope is
to use JniCodeEmitter as a base for emitting static OpenGL ES
bindings.