Fix build without tcpwrappers

* configure.ac (MU_TCPWRAP_LIBRARIES): New subst variable.
Always includes lib/libmutcpwrap.a.  If libwrap is present on
the system and required, it is included as well
* imap4d/Makefile.am (TCPWRAP_LIBRARIES): Replace with
MU_TCPWRAP_LIBRARIES.
* maidag/Makefile.am: Likewise.
* pop3d/Makefile.am: Likewise.

@@ -222,10 +222,14 @@ esac
 LIBS=$saved_LIBS
 
 if test "$status_tcpwrap" = "yes"; then
-  AC_SUBST(TCPWRAP_LIBRARIES, "\${top_builddir}/lib/libmutcpwrap.a $TCPWRAP_LIBRARIES -lwrap")
+  TCPWRAP_LIBRARIES="$TCPWRAP_LIBRARIES -lwrap"
   AC_DEFINE_UNQUOTED(WITH_LIBWRAP, 1,
                      [Define to 1 to use tcp wrappers.])
+else
+  # Provide placeholder functions only
+  TCPWRAP_LIBRARIES=
 fi
+AC_SUBST([MU_TCPWRAP_LIBRARIES], "\${top_builddir}/lib/libmutcpwrap.a $TCPWRAP_LIBRARIES")
 
 AC_ARG_ENABLE([pthread],
               AC_HELP_STRING([--disable-pthread],

@@ -79,7 +79,7 @@ imap4d_LDADD = \
  ${MU_LIB_AUTH}\
  @MU_AUTHLIBS@ \
  ${MU_LIB_MAILUTILS}\
- @SERV_AUTHLIBS@ @MU_COMMON_LIBRARIES@ @TCPWRAP_LIBRARIES@
+ @SERV_AUTHLIBS@ @MU_COMMON_LIBRARIES@ @MU_TCPWRAP_LIBRARIES@
 
 ## This kludge is necessary to correctly establish imap4d -> MU_AUTHLIBS
 ## dependencies. Automake stupidly refuses to include them.

@@ -51,7 +51,7 @@ maidag_LDADD = \
  @MU_COMMON_LIBRARIES@\
  $(LIBMU_DBM)\
  @DBMLIBS@\
- @TCPWRAP_LIBRARIES@
+ @MU_TCPWRAP_LIBRARIES@
 
 AM_CPPFLAGS = -I${top_srcdir} @MU_APP_COMMON_INCLUDES@ @GUILE_INCLUDES@ \
  @PYTHON_INCLUDES@

@@ -61,7 +61,7 @@ pop3d_LDADD = \
  @MU_COMMON_LIBRARIES@\
  ${LIBMU_DBM}\
  @DBMLIBS@\
- @TCPWRAP_LIBRARIES@
+ @MU_TCPWRAP_LIBRARIES@
 
 popauth_SOURCES = popauth.c
 popauth_LDADD = \