Instructions on debugging a mex file with gdb.

This can be done, but is painful as 1) Matlab loads mex files (really dlls) into
arbitrary locations.  gdb does not know where the dll was relocated to, and
instead blithely assumes the dll is loaded at its default address (typically
0x10000000).  However, the dll won't have been loaded there so debugging fails.

The process that works at present:

1) Give the dll a unique base address.  cygmex as provided here is set so that
compiling with -g sets the image base to 0x28000000, and this seems to be unique
enough that dlls are loaded there.  It is important that only one dll be
compiled with -g (unless a different image base is given for each).

2) Start Matlab, and execute the mex file.  Do this in a way that causes an
error return: typically, with no arguments the mex file should issue and error
message and return.  The dll is now loaded into memory so gdb can find it.

3) Execute ps -W to find the process id of the Matlab session (call this pid)

4) Start gdb.

5) Issue command   attach pid

6) Issue command   symbol-file <mexFunctionName>.dll

7) Issue command  l  mexFunction, repeat until source lines to break at are seen

8) b lineWhatever

and now debug as usual.

gdb REALLY needs to be able to trap on a dll load, and to determine the actual
image base of the dll.  sheesh.