Generalized auto- autoconf invocation to work atop linux.
Fix to get nspr building again.

I tried using various --host settings on linux, but they all led to
build issues that seemed to step from misinvocations of NSINSTALL from
the various makefiles.  I briefly tried to dissect this, since I saw
evidence of others running into the same error, namely:

../../../config/./nsinstall -R -m 444 /home/pnkfelix/Dev/Mozilla/iontrail-wip/nsprpub/pr/include/md/ ../../../dist/include/nspr
../../../config/./nsinstall: cannot make symbolic link /home/pnkfelix/Dev/Mozilla/iontrail-wip/objdir-dbg-nspr/dist/include/nspr/md: File exists
make[3]: *** [export] Error 1
make[3]: Leaving directory `/home/pnkfelix/Dev/Mozilla/iontrail-wip/objdir-dbg-nspr/pr/include/md'

but I eventually gave up and just stopped passing the --host option at
all, and used --enable-64bit instead to enable the 64-bit build.