How to specify environment variables inside GNU Global gtags.conf file

Question

https://stackoverflow.com/a/26725652/257924 specifies a helpful environment variable called GTAGSFORCECPP to force .h files to be treated as C++ (as otherwise, C++ class definitions are not added as symbols to the tags database). Ideally, I would not have to set that env var in my startup scripts, but have it be somehow expressed into a centrally located gtags.conf that can be shared with other developers in a network. The gtags manual (specifically this section) is worded in such a way as to lead me to be believe it is possible:

You can use the following environment variables as a string
variable. If these variables are set then environment variables of
the same name are set before command execution.

    GREP_COLOR
    GREP_COLORS
    GTAGSBLANKENCODE
    GTAGSCACHE
    GTAGSFORCECPP
    GTAGSGLOBAL
    GTAGSGTAGS
    GTAGSLIBPATH
    GTAGSLOGGING
    GTAGSTHROUGH
    GTAGS_OPTIONS
    HTAGS_OPTIONS
    MAKEOBJDIR
    MAKEOBJDIRPREFIX
    TMPDIR

The following variables are also available as a string
variable. By default, each of them has a directory path given by
the configure script.

  bindir
  datadir
  libdir
  localstatedir
  sysconfdir

However, I'm not sure where to express it. For instance adding it like this gives a parsing error, so this is not correct:

#
# Built-in parsers.
#
gtags:\
    :tc=common:\
    :tc=builtin-parser:\
    :GTAGSFORCECPP=1:

Really what I'm after is forcing all .h files to be treated as C++, so there is probably a better way to go about it.