vra: a Value-Range Analyzer built on top of Code Listener
Makefile: append 'vra' on the list of analyzers
vra: hook unit and overall tests on 'make check'
vra/tests-unit: allow to compile gtest on its own

The compilation failed if gtest/gtest.h was not installed on the system:

make -C gtest
make[1]: Entering directory `/home/kdudka/git/predator/vra/tests-unit/gtest'
g++ -c -pthread -o libgtest.a gtest-all.cc
gtest-all.cc:39:25: fatal error: gtest/gtest.h: No such file or directory
compilation terminated.
make[1]: *** [all] Error 1
make[1]: Leaving directory `/home/kdudka/git/predator/vra/tests-unit/gtest'
make: *** [gtest/libgtest.a] Error 2
vra/tests-overall: make it possible to use system GCC
vra: denote closed intervals by angle brackets
vra: rename LoopFounder -> LoopFinder
vra: minor white-space changes