Adding support for compiling against pppd-2.5.0 (current master)

[enaess]

Adding support for compiling against the most current pppd/master branch

• Fixing up configure.ac to support pppd > 2.5.0
• Adding AC_PROG_CPP, AC_PROG_EGREP, PKG_PROG_PKG_CONFIG
• Various other autotools fixes

Fixing up the nm-l2tp-pppd-plugin by adding another compatibility header file.