unknown versions in config.report on Linuxes

Two issues cause unknown versions to appear in config.report for me:

• $CPP is used as the preprocessor command, but it is not a canonical, documented autoconf variable, is not used anywhere else in configure.ac and it evaluates to nothing on some systems.
• On Linuxes that have decimal $CXX -dumpversion e.g. 10.2, the expr used to compare versions doesn't work. You may use this script to test:

#!/bin/sh

if expr 10.2 \> 5 > /dev/null; then
  echo it works!
else
  echo something is wrong :/
fi

Getting rid of the backslash works on Linux, but then fails on BSDs. test with -lt just fails on both. So a portable solution has to be done.

A fix is proposed.

Before:

$ cat config.report | grep VERSION
  CXX_VERSION:     g++ (GCC) 10.2.0
  PYTHON_VERSION:  3.9
  BOOST_VERSION:   unknown
  CRYPTO_VERSION:  unknown
  LOG4CPLUS_VERSION:  unknown
  MYSQL_VERSION:   10.5.8
  PGSQL_VERSION:   PostgreSQL 13.1
  CQL_VERSION:     2.7.1
  GTEST_VERSION:   unknown

After:

$ cat config.report | grep VERSION
  CXX_VERSION:     g++ (GCC) 10.2.0
  PYTHON_VERSION:  3.9
  BOOST_VERSION:   1.75
  CRYPTO_VERSION:  2.17.3
  LOG4CPLUS_VERSION:  2.0.5
  MYSQL_VERSION:   10.5.8
  PGSQL_VERSION:   PostgreSQL 13.1
  CQL_VERSION:     2.7.1
  GTEST_VERSION:   unknown