gcj -D

[Tom Tromey]

Following Anthony's change to have libgcj recognize the contents of
the GCJ_PROPERTIES environment variable at runtime, I've changed the
compiler to accept `-D' options.  A gcj `-D' option is just like C:
-DVAR[=VALUE].  Each such option is compiled into the main program
(unlike C you can only use `gcj -D' when you are also using --main).
These values are added to the system properties at runtime.

A couple final steps:

* Change `gij' to understand -D options (just like the JDK "java"
program does).

* Change the runtime to interpret some properties specially (e.g., add
a way to set the default heap size).

I don't have immediate plans to implement either of these.
Contributions welcome.

Tom