Find the absolute path of the top source directory and set it in
@SRCDIR@. Unfortunately the @top_src@ variable, as built into
autoconf, returns a relative path, what is not what I need.
M4 Source Code
AC_DEFUN([PETI_PATH_SRCDIR], [
AC_MSG_CHECKING(path of top source directory)
SRCDIR=`(cd $srcdir;pwd)`
AC_MSG_RESULT($SRCDIR)
AC_SUBST(SRCDIR)
])