bug#15880: Compute C declarations for DEFSYMs automatically.

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 12 Nov 2013 16:33:56 -0800
> From: Paul Eggert <eggert@cs.ucla.edu>
> 
>  globals.h: gl-stamp; @true
>  
> -GLOBAL_SOURCES = $(base_obj:.o=.c) $(NS_OBJC_OBJ:.o=.m)
> -
> -gl-stamp: $(libsrc)/make-docfile$(EXEEXT) $(GLOBAL_SOURCES)
> -	$(libsrc)/make-docfile -d $(srcdir) -g $(obj) > gl.tmp
> +# Set this to 'false' to speed the build but bloat Emacs a bit, by causing
> +# the gl-stamp build process to bypass the preprocessor and access
> +# symbols directly, even symbols that would be preprocessed away.
> +GLOBALS_CPP = $(CPP)
> +
> +GLOBALS_SOURCES = $(base_obj:.o=.c) $(NS_OBJC_OBJ:.o=.m)
> +
> +gl-stamp: $(libsrc)/make-docfile$(EXEEXT) $(GLOBALS_SOURCES)
> +	test -f globals.h || echo '/* dummy */' >globals.h
> +	rm -f ok.tmp
> +	($(GLOBALS_CPP) -DGENERATE_GLOBALS $(NONDEP_CFLAGS) \
> +	   $(base_obj:.o=.c) \
> +	 && { set x $(NS_OBJC_OBJ:.o=.m); shift; \
> +	      test $$# -eq 0 \
> +	      || $(GLOBALS_CPP) -DGENERATE_GLOBALS $(NONDEP_OBJC_CFLAGS) \
> +		   "$$@"; } \
> +	 && touch ok.tmp) | $(libsrc)/make-docfile -g >gl.tmp
> +	test -f ok.tmp || $(libsrc)/make-docfile -d $(srcdir) -g $(obj) >gl.tmp
>  	$(srcdir)/../build-aux/move-if-change gl.tmp globals.h
>  	echo timestamp > $@

Is this use of "{..}" in shell commands portable enough?