diff --git a/README b/README
index 4cfcd3c268da47484f842a8426d04efdb77330ee..325ade0c22522feb1c1689f957762ed760f7adf5 100644
--- a/README
+++ b/README
@@ -36,6 +36,10 @@ WARNING WARNING WARNING WARNING WARNING WARNING WARNING WARNING WARNING
     using pkg-config. If X11 is not found, some demo programs will
     not be able to display graphics and will fail.
 
+    Users with X11 installed in a non-standard location can specify
+    the paths to the development headers and library by passing the
+    X11_CFLAGS and X11_LIBS variables on the command line.
+
     If you  don't have X11  at all, fix  the definition of  the `EXES`
     variable as described in the top-level Makefile.
 
diff --git a/graph/x11/rules.mk b/graph/x11/rules.mk
index 9072f988c0e42f4fdbe41aab43df5e7f0fdfb288..e58c8e8a4912561ca0d51299e80dce262606f133 100644
--- a/graph/x11/rules.mk
+++ b/graph/x11/rules.mk
@@ -9,14 +9,14 @@ ifeq ($(PKG_CONFIG),)
   PKG_CONFIG = pkg-config
 endif
 
-X11_INCLUDE := $(shell $(PKG_CONFIG) --cflags x11)
-X11_LIB     := $(shell $(PKG_CONFIG) --libs x11)
+X11_CFLAGS ?= $(shell $(PKG_CONFIG) --cflags x11)
+X11_LIBS   ?= $(shell $(PKG_CONFIG) --libs x11)
 
-ifneq ($(X11_LIB),)
+ifneq ($(X11_LIBS),)
   # The GRAPH_LINK variable is expanded each time an executable is linked
   # against the graphics library.
   #
-  GRAPH_LINK += $(X11_LIB)
+  GRAPH_LINK += $(X11_LIBS)
 
   # Solaris needs a -lsocket in GRAPH_LINK.
   #
@@ -41,12 +41,12 @@ ifneq ($(X11_LIB),)
 	  $(LIBTOOL) --mode=compile $(CC) -static $(CFLAGS) \
                      $(GRAPH_INCLUDES:%=$I%) \
                      $I$(subst /,$(COMPILER_SEP),$(GR_X11)) \
-                     $(X11_INCLUDE:%=$I%) \
+                     $(X11_CFLAGS:%=$I%) \
                      $T$(subst /,$(COMPILER_SEP),$@ $<)
   else
 	  $(CC) $(CFLAGS) $(GRAPH_INCLUDES:%=$I%) \
                 $I$(subst /,$(COMPILER_SEP),$(GR_X11)) \
-                $(X11_INCLUDE:%=$I%) \
+                $(X11_CFLAGS:%=$I%) \
                 $T$(subst /,$(COMPILER_SEP),$@ $<)
   endif
 endif