[ODE] minor issues with ode 0.7

[David Walters]

> * fix_dTRIMESH_ENABLED_undef.diff:
> if ./configure is called with --disable-opcode, heightfield.cpp failed to build.

I'm submitted a fix for this, thanks for drawing it to my attention!

SVN #1078 : Fix for dTRIMESH_ENABLED support to allow "#define
dTRIMESH_ENABLED 0" to disable trimesh operation correctly. NOTE:
"#define dTRIMESH_ENABLED" is no longer supported, you must use
"#define dTRIMESH_ENABLED 1" to use trimeshes, as per the default
config file.

Regards,
Dave

ps. I've not looked at the other issue regarding the 'DESTDIR'
variable, so that is still an open problem.