Use GLOG_* environment variables even when gflags is installed.

Use GLOG_* environment variables even when gflags is installed.

Define GLOG_DEFINE_*, which checks if the GLOG_* environment variable
is defined. If defined, GLOG_DEFINE_* passes the value and otherwise,
it passes the original default value. In this way, GLOG_DEFINE_* macro
uses the value specified by gflags first, then GLOG_* environment
variables, and finally it uses the default value if neither of them are
specified.

git-svn-id: https://google-glog.googlecode.com/svn/trunk@76 eb4d4688-79bd-11dd-afb4-1d65580434c0