A GNU make rule is
  installed by default as
  /usr/share/libcxx- or
  version/libcxx.mk/usr/local/share/libcxx-,
  and contains
  a couple of useful gmake macros.
  This rule requires GNU
  make
  and xsltproc:
version/libcxx.mk
include /usr/share/libcxx-version/libcxx.mk $(call OPTIONS_GEN,cliopts.H,cliopts.xml)
      This example creates a rule for cliopts.H
      with a dependency cliopts.xml.
      The target gets created by running the
      optgen.xsl stylesheet.