Improve shell portability of new rules in Makefile.shared.

Makefile.shared

@@ -93,8 +93,8 @@ LINK_APP=	\
     LIBPATH=`for x in $$LIBDEPS; do if echo $$x | grep '^ *-L' > /dev/null 2>&1; then echo $$x | sed -e 's/^ *-L//'; fi; done | uniq`; \
     LIBPATH=`echo $$LIBPATH | sed -e 's/ /:/g'`; \
     LD_LIBRARY_PATH=$$LIBPATH:$$LD_LIBRARY_PATH \
-    $${LDCMD:-$(CC)} $${LDFLAGS:-$(CFLAGS)} \
-	-o $${APPNAME:-$(APPNAME)} $(OBJECTS) $$LIBDEPS )
+    $${LDCMD:=$(CC)} $${LDFLAGS:=$(CFLAGS)} \
+	-o $${APPNAME:=$(APPNAME)} $(OBJECTS) $$LIBDEPS )
 
 LINK_SO=	\
   ( $(SET_X);   \
@@ -103,7 +103,7 @@ LINK_SO=	\
     LIBPATH=`for x in $$LIBDEPS; do if echo $$x | grep '^ *-L' > /dev/null 2>&1; then echo $$x | sed -e 's/^ *-L//'; fi; done | uniq`; \
     LIBPATH=`echo $$LIBPATH | sed -e 's/ /:/g'`; \
     LD_LIBRARY_PATH=$$LIBPATH:$$LD_LIBRARY_PATH \
-    $${SHAREDCMD:-$(CC)} $${SHAREDFLAGS:-$(CFLAGS) $(SHARED_LDFLAGS)} \
+    $${SHAREDCMD:=$(CC)} $${SHAREDFLAGS:=$(CFLAGS) $(SHARED_LDFLAGS)} \
 	-o $$SHLIB$$SHLIB_SOVER$$SHLIB_SUFFIX \
 	$$ALLSYMSFLAGS $$SHOBJECTS $$NOALLSYMSFLAGS $$LIBDEPS \
   ) && $(SYMLINK_SO); \