Do not append ${prefix}/include to CPPFLAGS.

If configure didn't see it, neither should make. (closes: #2603)

--HG--
branch : HEAD

Makefile.am

@@ -54,8 +54,7 @@ DEFS=-DPKGDATADIR=\"$(pkgdatadir)\" -DSYSCONFDIR=\"$(sysconfdir)\" \
 
 AM_CPPFLAGS=-I. -I$(top_srcdir) $(IMAP_INCLUDES) $(GPGME_CFLAGS) -Iintl
 
-CPPFLAGS=@CPPFLAGS@ -I$(includedir)
-
+CPPFLAGS=@CPPFLAGS@
 
 EXTRA_mutt_SOURCES = account.c md5c.c mutt_sasl.c mutt_socket.c mutt_ssl.c \
 	mutt_tunnel.c pop.c pop_auth.c pop_lib.c smime.c pgp.c pgpinvoke.c pgpkey.c \