enabling optimizations (-Og) with --enable-debug

[Luke Deller]

Hi,

I saw that gcc-4.8 introduced a new optimization level -Og which enables 
optimizations that do not interfere with debugging.

Yesterday I floated the idea on IRC of using this with --enable-debug 
rather than -O0 if available, and some feedback was that it enables gcc 
to report extra warnings which would be a good thing.

I tried it out and can see that it does indeed break the build if 
--enable-werror is enabled, eg:

/home/deller/build/libo/vcl/unx/glxtest.cxx: In function ‘void glxtest()’:
/home/deller/build/libo/vcl/unx/glxtest.cxx:236:44: error: ignoring 
return value of ‘ssize_t write(int, const void*, size_t)’, declared with 
attribute warn_unused_result [-Werror=unused-result]
    write(write_end_of_the_pipe, buf, length);

So I guess this means we couldn't incorporate -Og until all these new 
warnings are fixed right?

Any other thoughts on whether this is a good idea to pursue?  One fear 
was that it might slow down compilation, but it doesn't actually seem to 
slow down "make clean && make" much at all for me.  I'll get some proper 
timing measurements.

(A draft patch is attached to enable -Og if available)

Regards,
Luke.
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