Ensure that library dependencies work across Makefiles

Since the main Makefile doesn't know the dependencies for the nolibc and
openlibm libraries, create intermediate phony targets.
That scheme ensures, even when one library already exists, that:
- when building a target that depend on that library, a recursive make
  is invoked to check that it is up-to-date,
- if the recursive make doesn't update the library, the dependency on
  the library is not a reason to rebuild the target.

Makefile

@@ -18,14 +18,24 @@ LIBS := openlibm/libopenlibm.a nolibc/libnolibc.a
 LIB_CFLAGS=-I$(TOP)/nolibc/include -include _solo5/overrides.h
 
 # NOLIBC
+# Use a phony target indirection, so that nolibc/Makefile is always checked to
+# see whether the library should be rebuilt while avoiding useless rebuild if
+# nolibc/libnolibc.a was up-to-date
 NOLIBC_CFLAGS=$(LIB_CFLAGS) -I$(TOP)/openlibm/src -I$(TOP)/openlibm/include
-nolibc/libnolibc.a:
+nolibc/libnolibc.a: phony-nolibc
+
+.PHONY: phony-nolibc
+phony-nolibc:
 	$(MAKE) -C nolibc libnolibc.a \
 	    "CC=$(MAKECONF_TOOLCHAIN)-cc" \
 	    "FREESTANDING_CFLAGS=$(NOLIBC_CFLAGS)"
 
 # OPENLIBM
-openlibm/libopenlibm.a:
+# See NOLIBC for explanations of the phony target
+openlibm/libopenlibm.a: phony-openlibm
+
+.PHONY: phony-openlibm
+phony-openlibm:
 	$(MAKE) -C openlibm libopenlibm.a \
 	     "CC=$(MAKECONF_TOOLCHAIN)-cc" \
 	     "CPPFLAGS=$(LIB_CFLAGS)"