refactor: libc - share printf, split the float path, fix relink deps

programs/GNUmakefile

@@ -105,8 +105,24 @@ lib/bearssl/build/libbearssl.a:
 bearssl: lib/bearssl/build/libbearssl.a
 
 # Build shared libc static library.
-libc:
-	$(MAKE) -C lib/libc
+#
+# The archives and CRT objects get a real file rule, not just the `libc`
+# phony, so that programs can list them as prerequisites and actually relink
+# when libc changes. A phony prerequisite would instead make every program
+# permanently out of date; no prerequisite at all (the old state) both races
+# under -j and silently leaves programs linked against a stale libc.
+# Grouped target (&:, GNU make 4.3+): one sub-make produces all of them.
+LIBC_DIR     := lib/libc
+LIBC_LIB     := $(LIBC_DIR)/liblibc.a
+LIBC_SRCS    := $(wildcard $(LIBC_DIR)/*.c $(LIBC_DIR)/*.h $(LIBC_DIR)/*.S \
+                           $(LIBC_DIR)/crt/*.c) $(LIBC_DIR)/Makefile
+LIBC_OUTPUTS := $(LIBC_LIB) $(LIBC_DIR)/liblibc-full.a \
+                $(LIBC_DIR)/crt1.o $(LIBC_DIR)/crti.o $(LIBC_DIR)/crtn.o
+
+$(LIBC_OUTPUTS) &: $(LIBC_SRCS)
+	$(MAKE) -C $(LIBC_DIR)
+
+libc: $(LIBC_OUTPUTS)
 
 # Build shared TLS helper library.
 tls: bearssl libc
@@ -144,8 +160,8 @@ libjpegwrite: libc
 imageviewer: libc libjpeg
 	$(MAKE) -C src/imageviewer
 
-# Build fontpreview standalone GUI client (no library deps).
-fontpreview:
+# Build fontpreview standalone GUI client (links libc).
+fontpreview: libc
 	$(MAKE) -C src/fontpreview
 
 # Build spreadsheet standalone GUI client (depends on libc and libdialogs).
@@ -271,8 +287,9 @@ mandelbrot: libc
 rpgdemo: libc
 	$(MAKE) -C src/rpgdemo
 
-# Build doom via its own Makefile (depends on libc).
-doom: libc
+# Build doom via its own Makefile (depends on libc and the shared-lib loader,
+# both of which it links).
+doom: libc libloader
 	$(MAKE) -C src/doom
 
 # Build screenshot tool (depends on libc, libjpegwrite, libloader, and libdialogs).
@@ -369,10 +386,10 @@ $(BINDIR)/os/wallpapers/%: $(WPDIR)/%
 	cp $< $@
 
 # Build each system program from its source files.
-$(BINDIR)/os/%.elf: src/%/main.cpp link.ld GNUmakefile
+$(BINDIR)/os/%.elf: src/%/main.cpp link.ld GNUmakefile $(LIBC_LIB)
 	mkdir -p $(BINDIR)/os obj/$*
 	$(CXX) $(CXXFLAGS) -c src/$*/main.cpp -o obj/$*/main.o
-	$(CXX) $(CXXFLAGS) $(LDFLAGS) obj/$*/main.o lib/libc/liblibc.a -o $@
+	$(CXX) $(CXXFLAGS) $(LDFLAGS) obj/$*/main.o $(LIBC_LIB) -o $@
 
 # ============================================================================
 # Native development kit (Montauk SDK): binutils built for MontaukOS
@@ -399,7 +416,10 @@ ifneq ($(wildcard $(NATIVE_BIN)/as),)
 	cp -r include/montauk $(BINDIR)/sdk/include/montauk
 	cp -r include/Api $(BINDIR)/sdk/include/Api
 	cp -r ../kernel/freestnd-cxx-hdrs/x86_64/include/. $(BINDIR)/sdk/include/
-	cp lib/libc/liblibc.a $(BINDIR)/sdk/lib/libc.a
+	# liblibc-full.a, not liblibc.a: on-OS builds cannot pass
+	# -Wl,-u,_pf_putfloat, so the sysroot ships the variant with the printf
+	# float path already bound in. See lib/libc/Makefile.
+	cp lib/libc/liblibc-full.a $(BINDIR)/sdk/lib/libc.a
 	cp lib/libc/crt1.o lib/libc/crti.o lib/libc/crtn.o $(BINDIR)/sdk/lib/
 	ar rcs $(BINDIR)/sdk/lib/libm.a
 	cp ../toolchain/local/x86_64-montauk/lib/libstdc++.a $(BINDIR)/sdk/lib/libstdc++.a