Fix race when building ada-lex.c

Prevent a race when building ada-lex.c, and any target of rules .c:.l or
.c:.y.  The target should be written only at the last step, else SIGINT
(^C) can leave an inconsistent state.  Being .PRECIOUS makes it even
worse.

gdb/ChangeLog:

	PR build/22873
	* gdb/Makefile.in: (.c:.l, .c:.y): Write the target only in the
	last step, and do it atomically.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2018-04-29  John Reiser  <jreiser@BitWagon.com>
+
+	PR build/22873
+	* gdb/Makefile.in: (.c:.l, .c:.y): Write the target only in the
+	last step, and do it atomically.
+
 2018-04-27  Alexandre Oliva  <aoliva@redhat.com>
 
 	* compile/compile-c-types.c (convert_int, convert_float):

gdb/Makefile.in

@@ -2483,9 +2483,8 @@ po/$(PACKAGE).pot: force
 # Makefile.in, but that was a pretty big annoyance.
 
 %.c: %.y
-	rm -f $@ $@.tmp
-	$(SHELL) $(YLWRAP) $< y.tab.c $@ -- $(YACC) $(YFLAGS) && mv $@ $@.tmp \
-		|| (rm -f $@; false)
+	$(SHELL) $(YLWRAP) $< y.tab.c $@.tmp -- $(YACC) $(YFLAGS) \
+		|| (rm -f $@.tmp; false)
 	sed -e '/extern.*malloc/d' \
 	     -e '/extern.*realloc/d' \
 	     -e '/extern.*free/d' \
@@ -2496,13 +2495,13 @@ po/$(PACKAGE).pot: force
 	     -e 's/\([ \t;,(]\)free$$/\1xfree/g' \
 	     -e '/^#line.*y.tab.c/d' \
 	     -e 's/YY_NULL/YY_NULLPTR/g' \
-	  < $@.tmp > $@
-	rm -f $@.tmp
+	  < $@.tmp > $@.new && \
+	  rm -f $@.tmp && \
+	  mv $@.new $@
 %.c: %.l
 	if [ "$(FLEX)" ] && $(FLEX) --version >/dev/null 2>&1; then \
-	    $(FLEX) -o$@ $< && \
-	    rm -f $@.new && \
-	    sed -e '/extern.*malloc/d' \
+	    $(FLEX) --stdout $<  \
+	    | sed -e '/extern.*malloc/d' \
 	        -e '/extern.*realloc/d' \
 	        -e '/extern.*free/d' \
 	        -e '/include.*malloc.h/d' \
@@ -2511,8 +2510,7 @@ po/$(PACKAGE).pot: force
 	        -e 's/\([ \t;,(]\)free\([ \t]*[&(),]\)/\1xfree\2/g' \
 	        -e 's/\([ \t;,(]\)free$$/\1xfree/g' \
 		-e 's/yy_flex_xrealloc/yyxrealloc/g' \
-	      < $@ > $@.new && \
-	    rm -f $@ && \
+	      > $@.new && \
 	    mv $@.new $@; \
 	elif [ -f $@ ]; then \
 	    echo "Warning: $*.c older than $*.l and flex not available."; \