Build system improvements followup issues

[wirew0rm]

After #142 was merged, there are some follow up improvements on the build system side which should be addressed in a followup PR:

• add a way to allow using system libraries (mostly only useful for zeromq, as the others are all header only)
  • relevant recent cmake improvements: dependency providers
• fix the install directives, find out whether and where to install dependencies
• fix clang build
• other compilers, OSes, WASM
• static linking?
• reenable -Werror (commented out) needs warnings resolved first
• disable gsl-lite warnings and bring SYSTEM includepath upstream in rxcpp
• Precompiled headers to decrease build time
• use gcovr from ubunut instead of pip? (as implemented already in graph-prototype)