Merge AM_CFLAGS and INCLUDES info AM_CPPFLAGS

AM_CPPFLAGS is for flags passed to the preprocessor, such as defines and
includes. AM_CFLAGS is for flags affecting the compiler, such as debug
and optimization settings.

INCLUDES is an obsolete name. Users can pass INCLUDES and break
compilation. AM_CPPFLAGS is more explicit that the flags come from
Automake and should not be overridden.

common/Makefile.am

@@ -19,7 +19,7 @@ EXTRA_DIST = \
   xrdp_rail.h \
   crc16.h
 
-AM_CFLAGS = \
+AM_CPPFLAGS = \
   -DXRDP_CFG_PATH=\"${sysconfdir}/xrdp\" \
   -DXRDP_SBIN_PATH=\"${sbindir}\" \
   -DXRDP_SHARE_PATH=\"${datadir}/xrdp\" \