Avoid re-copying include headers (#549)

This adds a `.stamp` file to the build directory that tracks when all
headers have been copied over to the sysroot `include` directory.
Previously, the `include_dirs` target was phony, which `make` will run
every time, regardless of whether it is needed. This was part of the
problem with all of libc being rebuilt at each `make` invocation.

Makefile

@@ -66,6 +66,10 @@ ifeq ($(WASI_SNAPSHOT), p2)
 TARGET_TRIPLE = wasm32-wasip2
 endif
 
+# These artifacts are "stamps" that we use to mark that some task (e.g., copying
+# files) has been completed.
+INCLUDE_DIRS := $(OBJDIR)/copy-include-headers.stamp
+
 BUILTINS_LIB ?= $(shell ${CC} ${CFLAGS} --print-libgcc-file-name)
 
 # These variables describe the locations of various files and directories in
@@ -735,31 +739,31 @@ $(LIBSETJMP_OBJS) $(LIBSETJMP_SO_OBJS): CFLAGS += \
 $(LIBWASI_EMULATED_SIGNAL_MUSL_OBJS) $(LIBWASI_EMULATED_SIGNAL_MUSL_SO_OBJS): CFLAGS += \
 	    -D_WASI_EMULATED_SIGNAL
 
-$(OBJDIR)/%.long-double.pic.o: %.c include_dirs
+$(OBJDIR)/%.long-double.pic.o: %.c $(INCLUDE_DIRS)
 	@mkdir -p "$(@D)"
 	$(CC) $(CFLAGS) -MD -MP -o $@ -c $<
 
 $(OBJDIR)/wasip2_component_type.pic.o $(OBJDIR)/wasip2_component_type.o: $(LIBC_BOTTOM_HALF_SOURCES)/wasip2_component_type.o
 	@mkdir -p "$(@D)"
 	cp $< $@
 
-$(OBJDIR)/%.pic.o: %.c include_dirs
+$(OBJDIR)/%.pic.o: %.c $(INCLUDE_DIRS)
 	@mkdir -p "$(@D)"
 	$(CC) $(CFLAGS) -MD -MP -o $@ -c $<
 
-$(OBJDIR)/%.long-double.o: %.c include_dirs
+$(OBJDIR)/%.long-double.o: %.c $(INCLUDE_DIRS)
 	@mkdir -p "$(@D)"
 	$(CC) $(CFLAGS) -MD -MP -o $@ -c $<
 
-$(OBJDIR)/%.no-floating-point.o: %.c include_dirs
+$(OBJDIR)/%.no-floating-point.o: %.c $(INCLUDE_DIRS)
 	@mkdir -p "$(@D)"
 	$(CC) $(CFLAGS) -MD -MP -o $@ -c $<
 
-$(OBJDIR)/%.o: %.c include_dirs
+$(OBJDIR)/%.o: %.c $(INCLUDE_DIRS)
 	@mkdir -p "$(@D)"
 	$(CC) $(CFLAGS) -MD -MP -o $@ -c $<
 
-$(OBJDIR)/%.o: %.s include_dirs
+$(OBJDIR)/%.o: %.s $(INCLUDE_DIRS)
 	@mkdir -p "$(@D)"
 	$(CC) $(ASMFLAGS) -o $@ -c $<
 
@@ -807,7 +811,10 @@ $(LIBWASI_EMULATED_PTHREAD_OBJS) $(LIBWASI_EMULATED_PTHREAD_SO_OBJS): CFLAGS +=
 $(EMMALLOC_OBJS): CFLAGS += \
     -fno-strict-aliasing
 
-include_dirs:
+ALL_POSSIBLE_HEADERS += $(shell find $(LIBC_TOP_HALF_MUSL_DIR) -name \*.h)
+ALL_POSSIBLE_HEADERS += $(shell find $(LIBC_BOTTOM_HALF_HEADERS_PUBLIC) -name \*.h)
+ALL_POSSIBLE_HEADERS += $(shell find $(MUSL_FTS_SRC_DIR) -name \*.h)
+$(INCLUDE_DIRS): $(ALL_POSSIBLE_HEADERS)
 	#
 	# Install the include files.
 	#
@@ -823,10 +830,12 @@ include_dirs:
 
 	# Copy in the bulk of musl's public header files.
 	cp -r "$(LIBC_TOP_HALF_MUSL_INC)"/* "$(SYSROOT_INC)"
+
 	# Copy in the musl's "bits" header files.
 	cp -r "$(LIBC_TOP_HALF_MUSL_DIR)"/arch/generic/bits/* "$(SYSROOT_INC)/bits"
 	cp -r "$(LIBC_TOP_HALF_MUSL_DIR)"/arch/wasm32/bits/* "$(SYSROOT_INC)/bits"
 
+	# Copy in the fts header files.
 	cp "$(MUSL_FTS_SRC_DIR)/fts.h" "$(SYSROOT_INC)/fts.h"
 
 	# Remove selected header files.
@@ -836,7 +845,11 @@ ifeq ($(WASI_SNAPSHOT), p2)
 		> "$(SYSROOT_INC)/__wasi_snapshot.h"
 endif
 
-startup_files: include_dirs $(LIBC_BOTTOM_HALF_CRT_OBJS)
+	# Stamp the include installation.
+	@mkdir -p $(@D)
+	touch $@
+
+startup_files: $(INCLUDE_DIRS) $(LIBC_BOTTOM_HALF_CRT_OBJS)
 	#
 	# Install the startup files (crt1.o etc).
 	#
@@ -861,7 +874,7 @@ LIBC_SO += \
 endif
 endif
 
-libc_so: include_dirs $(LIBC_SO)
+libc_so: $(INCLUDE_DIRS) $(LIBC_SO)
 
 STATIC_LIBS = \
     $(SYSROOT_LIB)/libc.a \
@@ -881,7 +894,7 @@ STATIC_LIBS += \
 	$(SYSROOT_LIB)/libsetjmp.a
 endif
 
-libc: include_dirs $(STATIC_LIBS)
+libc: $(INCLUDE_DIRS) $(STATIC_LIBS)
 
 DUMMY := m rt pthread crypt util xnet resolv
 DUMMY_LIBS := $(patsubst %,$(SYSROOT_LIB)/lib%.a,$(DUMMY))
@@ -1101,4 +1114,4 @@ clean:
 	$(RM) -r "$(OBJDIR)"
 	$(RM) -r "$(SYSROOT)"
 
-.PHONY: default startup_files libc libc_so finish install include_dirs clean check-symbols bindings
+.PHONY: default startup_files libc libc_so finish install clean check-symbols bindings