When compiling with clang we get:

src/VBox/Devices/PC/ipxe/src/core/settings.c:310:8: error: fields must have a constant size: 'variable length array in structure' extension will never be supported

Apply this patch conditionally to disable rebuilding iPxeBiosBin.rom when using clang.

--- a/src/VBox/Devices/PC/ipxe/Makefile.kmk
+++ b/src/VBox/Devices/PC/ipxe/Makefile.kmk
@@ -45,7 +45,8 @@
 iPxeBiosBin_CLEAN    = \
 	$(iPxeBiosBin_0_OUTDIR)/iPxeBiosBin.c \
 
-if1of ($(KBUILD_TARGET), darwin os2 solaris win)
+# Disable building on Linux if using clang
+if1of ($(KBUILD_TARGET), darwin os2 solaris win linux)
  $$(iPxeBiosBin_0_OUTDIR)/iPxeBiosBin.c: $(PATH_SUB_CURRENT)/iPxeBiosBin.rom $(VBOX_BIN2C) | $$(dir $$@)
 	$(call MSG_TOOL,bin2c,iPxeBiosBin,$<,$@)
 	$(QUIET)$(VBOX_BIN2C) -min 32 -max 56 -mask 0x1ff -export NetBiosBinary $< $@