pipeline that takes stdin, performs the substitions and writes the result
to stdout. It defaults to ${SED} ${SUBST_SED.<class>}. Use this variable
to replace the sed command with something else, like an awk process.
facility for different classes of files in ${WRKSRC}. For each class of
files, a target <class>-subst is created to perform the text replacement.
The following variables are used:
SUBST_STAGE.<class>
"stage" at which we do the text replacement, e.g. pre-configure,
post-build, etc.
SUBST_MESSAGE.<class>
message to display, noting what is being substituted
SUBST_FILES.<class>
files on which to run the substitution; these are relative to
${WRKSRC}
SUBST_SED.<class>
sed(1) substitution expression to run on the specified files
This basically extracts a useful piece of code from bsd.buildlink2.mk and
puts it in a place that allows it to be more widely used, and so that the
functionality doesn't depend on USE_BUILDLINK2 being defined.
