Compile the VM code with some optimizations (+130% speedup).

Option -g has no impact on the code generated by GCC, so it is now
systematically added, since it is quite helpful when the VM segfaults
(e.g. bug #4203). Note that, even for a debug build, option -O1 is
preferred to -O0, since -O0 produces assembly code that is much too noisy
for debugging.

For non-debugging builds, -O2 was chosen rather than -O3, since it led to
a noticeably faster VM. I guess even recent GCC compilers still have a
hard time optimizing humongous functions such as the VM.

Here are the results on a simple benchmark: computing the factorial of a
large number with Z and BigZ. (Factorial of 2*6! for Z, of 7! for BigZ.)
For comparison purpose, the timings of native_compute are also provided.

      Z     BigZ
-O0   6.4   12.3
-O1   4.3   10.7
-O2   2.8    7.3
-O3   3.0    9.3
n_c   2.0    2.4

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