much more reliable REQUIRE_TEST detection with the old method. only parts that are optimized out completely by the linker cannot be found this way

- rule `doc` in ./Makefile depended on `doc/parseDoxytags` which had no rule
  in ./Makefile
- add missing `-std=c++11` flag to `$(CXX)`-calls in ./doc/Makefile
- replaced .FORCE rules by .PHONY declarations

The declaration of those variables as static const class members caused
problems in codes, that did not define exactly those variables in their
code. This behaviour was highly unexpected.

less namespace pollution (relates #182): all exported and used macros now use the XERUS_ prefix. LogFlag now in xerus::misc::internal