Bug 1641775 - Move --with-system-nspr to python configure....

Bug 1641775 - Move --with-system-nspr to python configure. r=firefox-build-system-reviewers,rstewart

Versions of NSPR >= 4.10 come with a pkg-config file. We currently
depend on 4.9.2 for spidermonkey, but much more recent versions for
Firefox. 4.10 is less than a year newer than 4.9.2, and 4.10 is 7 years
old, so bumping the requirement to 4.10 is not really a big deal.

With the use of pkg-config, --with-nspr-cflags and --with-nspr-libs are
not needed.

None of the AC_TRY_COMPILE tests were any useful because
PR_STATIC_ASSERT and PR_UINT64 have been when we look for them since
4.8.6 and 4.9 respectively.

Differential Revision: https://phabricator.services.mozilla.com/D77412