libweston: fix building issue when EGL support is not enabled

weston-egl-ext.h has been include in compositor-xx.c file in order to
define EGL_PLATFORM_xxx_KHR extensions used by the compositors.
But in case EGL support is not enabled, all EGL related definition must
be skipped except EGL_PLATFORM_xxx_KHR that must be still defined to
allow compositor-xx.c to build.

Signed-off-by: Vincent Abriou <vincent.abriou@st.com>
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>

libweston/weston-egl-ext.h

@@ -28,6 +28,8 @@
 #ifndef WESTON_EGL_EXT_H
 #define WESTON_EGL_EXT_H
 
+#ifdef ENABLE_EGL
+
 #ifndef EGL_WL_bind_wayland_display
 #define EGL_WL_bind_wayland_display 1
 
@@ -152,5 +154,13 @@ typedef EGLSurface (EGLAPIENTRYP PFNEGLCREATEPLATFORMPIXMAPSURFACEEXTPROC) (EGLD
 #define EGL_PLATFORM_X11_KHR 0x31D5
 #endif
 
+#else /* ENABLE_EGL */
+
+/* EGL platform definition are keept to allow compositor-xx.c to build */
+#define EGL_PLATFORM_GBM_KHR     0x31D7
+#define EGL_PLATFORM_WAYLAND_KHR 0x31D8
+#define EGL_PLATFORM_X11_KHR     0x31D5
+
+#endif /* ENABLE_EGL */
 
 #endif