Replace non-portable make rule with one that is also easier to understand

Fixes:
warning: '%'-style pattern rules are a GNU make extension

src/Makefile.am

@@ -27,17 +27,11 @@ libhamlib_la_LIBADD = $(top_builddir)/lib/libmisc.la $(top_builddir)/security/li
 
 libhamlib_la_DEPENDENCIES = $(top_builddir)/lib/libmisc.la $(top_builddir)/security/libsecurity.la $(BACKENDEPS) $(RIG_BACKENDEPS) $(ROT_BACKENDEPS) $(AMP_BACKENDEPS) 
 
-# Rule to build the dependencies of libhamlib.la that are not in this directory
-# when running make in this directory instead of top_builddir
-# (the % matches any number of subdirectories)
-# warning: '%'-style pattern rules are a GNU make extension
-$(top_builddir)/%.la:
-	$(MAKE) -C $(@D) $(@F)
-
-#
-
 EXTRA_DIST = Android.mk hamlibdatetime.h.in band_changed.c
 
+# Rule to build the dependencies of libhamlib.la that are in other directories
+$(libhamlib_la_DEPENDENCIES):
+	$(MAKE) -C $(@D) $(@F)
 
 # If we have  a .git directory then we will  generate the hamlibdate.h
 # file and  replace it if it  is different. Fall  back to a copy  of a