[fortran, patch] Fix handling of backtrace options in the library

[FX]

Hi all,

I'll offer my first patch to the new 4.8 trunk. I noticed that the -fbacktrace option and the GFORTRAN_ERROR_BACKTRACE environment variable are somewhat inconsistently handled. Currently, the environment variabled doesn't actually override the compilation option, as it should. So, I set to change that (checking both options.backtrace and compile_options.backtrace instead of only the latter) when I realized that we could improve this further by moving the checking code: it is currently located in the set_options() function, which is called from the main Fortran routine (call generated by the front-end). This means that it's not triggered if the main program is C. So, by moving it in the main initialization routine, we can ensure that an executable with a C main program but run using GFORTRAN_ERROR_BACKTRACE=Y will actually generate backtraces for Fortran errors, which I think is the cool thing to do.

Full disclosure: I removed the function maybe_find_addr2line() because it's not needed anymore now the code was regrouped. However, I'll note that I don't understand why it was useful before, and that I think the comment on top of it was wrong (variable "options" could be seen in the function, it was just shadowed by the local function argument!). Thus, while I was at it, I renamed the options argument to set_options(), just to make sure we didn't have a problem of this sort in the future.

The change was bootstrapped and regtested on x86_64-apple-darwin11, but is an area not covered by the testsuite. I have manually checked that running various types of abort, both in a pure Fortran code and a mixed C/Fortran (with C main), with all combinations -fno-backtrace/GFORTRAN_ERROR_BACKTRACE, behaved as expected.

OK to commit to trunk?
FX

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