rtl_test: Support PPM error measurement on Windows.
rtl_fm: Support output padding on Windows.
convenience: No junk when device info isn't properly extracted.
rtl_fm: Avoid possible null pointer dereference.
Variable cleanup and corrections.

Remove unused variables.
Use proper type specifier.
Explicitly cast to correct type to suppress MSVC warnings.
build: Fix compilation on VS2013, Cygwin and MinGW.

- Link librtlsdr dynamically also on Windows.
build: libusb pkgconfig for NixOS

From Bjørn Forsman