build: fix configure when PKG_CONFIG is set

If you define the environment variable PKG_CONFIG and assign it to
pkgconf (or pkg-config), configure will say it's OK, but it won't be
used.
build: add required libraries to libuv.pc.in
Windows: improve timer precision

Improving timing precision by using QueryPerformanceCounter, whenever
a fast implementation is available.

This is part of the fix for Node.js' test-timers-first-fire.js.