Get more detailed GTEST_VERSION when using --with-gtest-source

It seems that extracting GTEST_VERSION doesn't work correctly with recent versions of GTest. config.report often shows:

Google Test:
  GTEST_VERSION:   googletest

Correct version could be extracted from sources - file CMakeLists.txt:

set(GOOGLETEST_VERSION 1.11.0)

e.g.:

GTEST_VERSION=`grep "set(GOOGLETEST_VERSION" -h $GTEST_SOURCE/* 2>/dev/null | cut -d' ' -f2 | sed 's/)//'`