kbuild: fix make headers_check with make 3.80

Commit 10b63956 ("UAPI: Plumb the UAPI Kbuilds into the user header
installation and checking") introduced a dependency of make 3.81
due to use of $(or ...)

We do not want to lift the requirement to gmake 3.81 just yet...
Included are a straightforward conversion to $(if ...)
Bisected-and-tested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: <stable@vger.kernel.org> [v3.7+]
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/Makefile.headersinst

@@ -14,7 +14,7 @@ kbuild-file := $(srctree)/$(obj)/Kbuild
 include $(kbuild-file)
 
 # called may set destination dir (when installing to asm/)
-_dst := $(or $(destination-y),$(dst),$(obj))
+_dst := $(if $(destination-y),$(destination-y),$(if $(dst),$(dst),$(obj)))
 
 old-kbuild-file := $(srctree)/$(subst uapi/,,$(obj))/Kbuild
 ifneq ($(wildcard $(old-kbuild-file)),)
@@ -48,13 +48,14 @@ all-files     := $(header-y) $(genhdr-y) $(wrapper-files)
 output-files  := $(addprefix $(installdir)/, $(all-files))
 
 input-files   := $(foreach hdr, $(header-y), \
-		   $(or \
+		   $(if $(wildcard $(srcdir)/$(hdr)), \
 			$(wildcard $(srcdir)/$(hdr)), \
-			$(wildcard $(oldsrcdir)/$(hdr)), \
-			$(error Missing UAPI file $(srcdir)/$(hdr)) \
+			$(if $(wildcard $(oldsrcdir)/$(hdr)), \
+				$(wildcard $(oldsrcdir)/$(hdr)), \
+				$(error Missing UAPI file $(srcdir)/$(hdr))) \
 		   )) \
 		 $(foreach hdr, $(genhdr-y), \
-		   $(or \
+		   $(if	$(wildcard $(gendir)/$(hdr)), \
 			$(wildcard $(gendir)/$(hdr)), \
 			$(error Missing generated UAPI file $(gendir)/$(hdr)) \
 		   ))