Help clang static analyzer to identify shogun's custom assertation
functions in order to avoid false positive reports
Simplify clang static analyzer helper
CLANG_ANALYZER_NORETURN caused some errors with gcc hence removing it
and use __attribute__((analyzer_noreturn)) for _clang_fail() when doing
static analysis
Fix version detection in cmake

- use currenlty active branch when determining merge-revision
- when .git is not available use mtime of NEWS file for dateinfo