It doesn't even depend on ELOOP, it just depends on the libc, so let's
claim a distinct config variable for this since we generally don't need
this dependency.

--- ./hostapd/Makefile.dist	2018-03-15 12:24:47.524745887 +0100
+++ ./hostapd/Makefile	2018-03-15 12:26:05.239539933 +0100
@@ -130,7 +130,7 @@
 OBJS += ../src/utils/$(CONFIG_ELOOP).o
 OBJS_c += ../src/utils/$(CONFIG_ELOOP).o
 
-ifeq ($(CONFIG_ELOOP), eloop)
+ifdef CONFIG_NEED_LIBRT
 # Using glibc < 2.17 requires -lrt for clock_gettime()
 LIBS += -lrt
 LIBS_c += -lrt