Bill Somerville authored Jul 29, 2015 and Brad King committed Jul 31, 2015

As dumpbin.exe is no longer reliable for gcc libraries on MinGW because
it crashes on many common libraries like libgcc_s and libgfortran it is
now necessary too resort to using objdump for DLL dependency walking.
Using objdump has a secondary problem in that it generates a lot of
output for large libraries and causes fixup_bundle() to take many
minutes to process what took fractions of a second with
"dumpbin.exe /dependents".

Add a 'grep' pre-filter in the execute_process() command pipeline to
reduce this output to a minimum for a several orders of magnitude speed
up.  If grep is not available just use the full output.

As there does not seem to be a reliable way of detecting MinGW, callers
of fixup_bundle() may have to set the variable gp_tool to "objdump" if
dumpbin.exe is installed on the build machine to stop it using the
broken MS dumpbin.exe for library dependency walking.