diff --git a/applications/ccdpack/Makefile.am b/applications/ccdpack/Makefile.am
index 6b4eeb683ba..95b963f6729 100644
--- a/applications/ccdpack/Makefile.am
+++ b/applications/ccdpack/Makefile.am
@@ -17,6 +17,7 @@ noinst_LTLIBRARIES = libgrf_ccdpack.la
 libgrf_ccdpack_la_SOURCES = grf_ccdpack.c grf.h
 
 ccdwish_SOURCES = ccdwish.c
+ccdwish_CFLAGS = $(X_CFLAGS)
 
 STARTCL_OBJ = $(STARLINK)/lib/tkGwm.o $(STARLINK)/lib/tkGwmCanv.o \
               $(STARLINK)/lib/tkGwmPrint.o $(STARLINK)/lib/tclAdam.o
diff --git a/applications/ccdpack/main/Makefile.am b/applications/ccdpack/main/Makefile.am
index d113baf4300..092f0eb7226 100644
--- a/applications/ccdpack/main/Makefile.am
+++ b/applications/ccdpack/main/Makefile.am
@@ -2,7 +2,7 @@ noinst_LTLIBRARIES = libccdpack.la
 libccdpack_la_SOURCES = $(SUBSRC) $(CSUBSRC)
 
 AM_FCFLAGS = -I..
-AM_CFLAGS = -I..
+AM_CFLAGS = -I.. $(X_CFLAGS)
 
 #  List of applications/tasks in monoliths.
 
diff --git a/libraries/gks/configure.ac b/libraries/gks/configure.ac
index 546385125c1..ead5c5a2842 100644
--- a/libraries/gks/configure.ac
+++ b/libraries/gks/configure.ac
@@ -33,12 +33,21 @@ dnl    Declare the build and use dependencies for this package
 STAR_DECLARE_DEPENDENCIES(build, [cnf gwm])
 STAR_DECLARE_DEPENDENCIES(link,  [ems gwm starx])
 
-dnl    Terminal control
+
 AC_HEADER_STDC
-AC_PATH_XTRA
-AC_CHECK_HEADERS([termios.h termio.h sgtty.h sys/ioctl.h X11/DECWmHints.h X11/Xlib.h])
+
+dnl    Terminal control
+AC_CHECK_HEADERS([termios.h termio.h sgtty.h sys/ioctl.h])
 AC_CHECK_FUNCS([isatty ioctl])
 
+dnl    Check for X11
+AC_PATH_XTRA
+
+dnl    Having no X11 is fatal.
+if test "$no_x" = "yes"; then
+   AC_MSG_ERROR([No X11 development system found. Cannot continue])
+fi
+
 dnl    Fortran features (mostly for ./base/*)
 AC_FC_OPEN_SPECIFIERS(readonly)
 AC_FC_RECL_UNIT
diff --git a/libraries/gks/drivers/xwindows/Makefile.am b/libraries/gks/drivers/xwindows/Makefile.am
index e78dfabd645..152562e5742 100644
--- a/libraries/gks/drivers/xwindows/Makefile.am
+++ b/libraries/gks/drivers/xwindows/Makefile.am
@@ -1,5 +1,7 @@
 
 ## Makefile simply needs to compile the files for later linking
+AM_CFLAGS = $(X_CFLAGS)
+
 noinst_LTLIBRARIES = libgks_driver_xwindows.la
 
 libgks_driver_xwindows_la_SOURCES = gk0xcs.f gk0xgi.f gk0xro.f \
diff --git a/libraries/gks/drivers/xwindows/x.c b/libraries/gks/drivers/xwindows/x.c
index a1acc6f0b65..933ac065bab 100644
--- a/libraries/gks/drivers/xwindows/x.c
+++ b/libraries/gks/drivers/xwindows/x.c
@@ -24,14 +24,9 @@
 
 #include <stdio.h>
 
-/* Assume we have all X11 includes if Xlib.h is there */
-#if HAVE_X11_XLIB_H
-# include <X11/Xlib.h>
-# include <X11/Xutil.h>
-# include <X11/keysym.h>
-#else
-# error "X11 include files must be installed"
-#endif
+#include <X11/Xlib.h>
+#include <X11/Xutil.h>
+#include <X11/keysym.h>
 
 #if defined(VMS)
 #  include "gwm_dir:gwm.h"
diff --git a/libraries/gwm/Makefile.am b/libraries/gwm/Makefile.am
index dd920f463c7..659b7580789 100644
--- a/libraries/gwm/Makefile.am
+++ b/libraries/gwm/Makefile.am
@@ -15,6 +15,7 @@ xdestroy_LDADD = libgwm.la $(X_LIBS) $(X_PRE_LIBS) -lX11 -lXt `ems_link` `cnf_li
 gwmXrefresh_SOURCES = gwmXrefresh.c
 gwmXrefresh_LDADD = libgwm.la $(X_LIBS) $(X_PRE_LIBS) -lX11 -lXt `ems_link` `cnf_link`
 
+AM_CFLAGS = $(X_CFLAGS)
 
 lib_LTLIBRARIES = libgwm.la
 
diff --git a/libraries/idi/Makefile.am b/libraries/idi/Makefile.am
index a4751d7cbff..c63d72dbd51 100644
--- a/libraries/idi/Makefile.am
+++ b/libraries/idi/Makefile.am
@@ -24,6 +24,8 @@ staretc_DATA = xworks.dat xworks.dct
 
 stardocs_DATA = @STAR_LATEX_DOCUMENTATION@
 
+AM_CFLAGS = $(X_CFLAGS)
+
 PUBLIC_INCLUDES = IDI_PAR idi.h
 
 PRIVATE_INCLUDES = idistruct_e.h kwm.h vdm.h idi_did \
diff --git a/libraries/idi/configure.ac b/libraries/idi/configure.ac
index cba43c6bc56..1b3f927c849 100644
--- a/libraries/idi/configure.ac
+++ b/libraries/idi/configure.ac
@@ -25,6 +25,11 @@ AC_PATH_XTRA
 AC_PROG_LIBTOOL
 AC_C_BIGENDIAN
 
+dnl    Having no X11 is fatal.
+if test "$no_x" = "yes"; then
+   AC_MSG_ERROR([No X11 development system found. Cannot continue])
+fi
+
 AC_CHECK_HEADERS([X11/decwmhints.h])
 AC_CHECK_HEADERS([unixlib.h])