diff --git a/BasiliskII/src/Unix/Makefile.in b/BasiliskII/src/Unix/Makefile.in
index 42f1b049..6d04f350 100644
--- a/BasiliskII/src/Unix/Makefile.in
+++ b/BasiliskII/src/Unix/Makefile.in
@@ -21,7 +21,7 @@ CFLAGS = @CFLAGS@
 CXXFLAGS = @CXXFLAGS@
 CPUINCLUDES_FLAGS=@CPUINCLUDES@
 CPUINCLUDES_FLAGS:=$(CPUINCLUDES_FLAGS:-I%=-I@top_srcdir@/%)
-CPPFLAGS = @CPPFLAGS@ -I@top_srcdir@/../include -I@top_srcdir@/. -I. -I@top_srcdir@/../CrossPlatform $(CPUINCLUDES_FLAGS) -I@top_srcdir@/../slirp
+CPPFLAGS = @CPPFLAGS@ -I/opt/X11/include -I@top_srcdir@/../include -I@top_srcdir@/. -I. -I@top_srcdir@/../CrossPlatform $(CPUINCLUDES_FLAGS) -I@top_srcdir@/../slirp
 DEFS = @DEFS@ @DEFINES@ -D_REENTRANT -DDATADIR=\"$(datadir)/$(APP)\"
 LDFLAGS = @LDFLAGS@
 LIBS = @LIBS@
diff --git a/BasiliskII/src/Unix/main_unix.cpp b/BasiliskII/src/Unix/main_unix.cpp
index 4243744b..e1622758 100755
--- a/BasiliskII/src/Unix/main_unix.cpp
+++ b/BasiliskII/src/Unix/main_unix.cpp
@@ -63,7 +63,9 @@ struct sigstate {
 # ifdef HAVE_GNOMEUI
 #  include <gnome.h>
 # endif
-# include <X11/Xlib.h>
+# ifndef GDK_WINDOWING_WAYLAND
+#  include <X11/Xlib.h>
+# endif
 #endif
 
 #ifdef ENABLE_XF86_DGA
@@ -385,7 +387,7 @@ static void usage(const char *prg_name)
 
 int main(int argc, char **argv)
 {
-#ifdef ENABLE_GTK
+#if defined(ENABLE_GTK) && !defined(GDK_WINDOWING_WAYLAND)
 	XInitThreads();
 #endif
 	const char *vmdir = NULL;
diff --git a/SheepShaver/src/Unix/Makefile.in b/SheepShaver/src/Unix/Makefile.in
index ed6148a4..f190e0ad 100644
--- a/SheepShaver/src/Unix/Makefile.in
+++ b/SheepShaver/src/Unix/Makefile.in
@@ -17,7 +17,7 @@ CC = @CC@
 CXX = @CXX@
 CFLAGS = @CFLAGS@
 CXXFLAGS = @CXXFLAGS@
-CPPFLAGS = @CPPFLAGS@ -I../include -I. -I../CrossPlatform -I../slirp
+CPPFLAGS = @CPPFLAGS@ -I/opt/X11/include -I../include -I. -I../CrossPlatform -I../slirp
 DEFS = @DEFS@ -D_REENTRANT -DDATADIR=\"$(datadir)/$(APP)\"
 LDFLAGS = @LDFLAGS@
 LIBS = @LIBS@
diff --git a/SheepShaver/src/Unix/main_unix.cpp b/SheepShaver/src/Unix/main_unix.cpp
index ad352607..98920bc0 100755
--- a/SheepShaver/src/Unix/main_unix.cpp
+++ b/SheepShaver/src/Unix/main_unix.cpp
@@ -133,8 +133,10 @@
 
 #ifdef ENABLE_GTK
 #include <gtk/gtk.h>
+#ifndef GDK_WINDOWING_WAYLAND
 #include <X11/Xlib.h>
 #endif
+#endif
 
 #ifdef ENABLE_XF86_DGA
 #include <X11/Xlib.h>
@@ -729,7 +731,7 @@ static bool init_sdl()
 
 int main(int argc, char **argv)
 {
-#ifdef ENABLE_GTK
+#if defined(ENABLE_GTK) && !defined(GDK_WINDOWING_WAYLAND)
 	XInitThreads();
 #endif
 	char str[256];