BuildSystem: Avoid $EUID and use id -u instead.

Makefile included a call to ${EUID} which requires the shell bash. To keep
compatibility with other shells like dash or ksh, use id -u instead.

ASTERISK-27589

Change-Id: Ia6e74f5bc9aab4e6dc62b7439f647b7964e6f657

Makefile

@@ -625,7 +625,7 @@ oldmodcheck:
 ld-cache-update:
 ifneq ($(LDCONFIG),)
 ifeq ($(DESTDIR),)  # DESTDIR means binary archive creation; ldconfig should be run on postinst
-	@if [ $${EUID} -eq 0 ] ; then \
+	@if [ $$(id -u) -eq 0 ] ; then \
 		$(LDCONFIG) "$(ASTLIBDIR)/" ; \
 	else \
 		echo " WARNING WARNING WARNING" ;\