build system: stop creating ugly double-slash symlinks like

fenv.h -> ../../libc/sysdeps/linux/i386/bits//fenv.h

Makefile.in

@@ -54,7 +54,10 @@ export header_extra_args = -n
 endif
 HEADERS_BITS_COMMON := $(notdir $(wildcard $(top_srcdir)libc/sysdeps/linux/common/bits/*.h))
 HEADERS_BITS_ARCH   := $(notdir $(wildcard $(top_srcdir)libc/sysdeps/linux/$(TARGET_ARCH)/bits/*.h))
-HEADERS_BITS_SUBARCH   := $(notdir $(wildcard $(top_srcdir)libc/sysdeps/linux/$(TARGET_ARCH)/bits/$(TARGET_SUBARCH)/*.h))
+HEADERS_BITS_SUBARCH :=
+ifneq ($(TARGET_SUBARCH),)
+HEADERS_BITS_SUBARCH := $(notdir $(wildcard $(top_srcdir)libc/sysdeps/linux/$(TARGET_ARCH)/bits/$(TARGET_SUBARCH)/*.h))
+endif
 HEADERS_BITS_COMMON := $(filter-out $(HEADERS_BITS_ARCH) $(HEADERS_BITS_SUBARCH),$(HEADERS_BITS_COMMON))
 headers: include/bits/uClibc_config.h
 	$(Q)$(MAKE) headers-y