shells/zsh: Fix runtime error when built by gcc-5 with upstream patch

ZSH was misbuilt on gcc-5 due to an intentional change in behavior of
gcc5's cpp.  This patch to execute cpp with the -P switch comes from
upstream.  No revbump necessary as GCC5 isn't used by default on FreeBSD.

Reported by:	DragonFly ZSH users
Approved by:	maintainer (bapt, IRC)

shells/zsh/files/patch-Src_zsh.mdd

@@ -0,0 +1,22 @@
+--- Src/zsh.mdd.orig	2014-09-20 19:01:46 UTC
++++ Src/zsh.mdd
+@@ -21,9 +21,18 @@ hdrdeps="zshcurses.h zshterm.h"
+ :<<\Make
+ @CONFIG_MK@
+ 
++# If we're using gcc as the preprocessor, get rid of the additional
++# lines generated by the preprocessor as they can confuse the script.
++# We don't need these in other cases either, but can't necessarily rely
++# on the option to remove them being the same.
+ signames.c: signames1.awk signames2.awk ../config.h @SIGNAL_H@
+ 	$(AWK) -f $(sdir)/signames1.awk @SIGNAL_H@ >sigtmp.c
+-	$(CPP) sigtmp.c >sigtmp.out
++	case "$(CPP)" in \
++	gcc*) \
++	$(CPP) -P sigtmp.c >sigtmp.out;; \
++	*) \
++	$(CPP) sigtmp.c >sigtmp.out;; \
++	esac
+ 	$(AWK) -f $(sdir)/signames2.awk sigtmp.out > $@
+ 	rm -f sigtmp.c sigtmp.out
+