Don't use PYTHON_VERSION_STRING when searching for Python libraries

It may be an arbitrary string, and, for example, in Ubuntu 13.10
it's set to "2.7.5+", which breaks the find script.
Merge pull request #2136 from SpecLad:python-ver
deleted extra arg in case of power=0.5
Merge pull request #2147 from ilya-lavrenov:tapi_sqrt