winpr: always build "monolitic"

winpr is now always build as single library.
The build option MONOLITHIC_BUILD doesn't influence this behavior anymore.

The only exception is winpr-makecert-tool which is still build as extra
library.

This obsoletes complex_libraries for winpr.
winpr: fix handling of absolute include paths

Don't prefix include paths if it is absolute
Merge pull request #1956 from bmiklautz/feat/monolithic

monolithic winpr