Preserve timestamps for installed headers to improve incremental builds

This change makes it so that the timestamps of installed header files
are only updated when the contents of the header files change.
This change is beneficial for incremental builds of projects depending
on wasi-libc. For example, the Swift project builds wasi-libc as part
of its build process, and this change reduces the number of build
products in the Swift build that are unnecessarily rebuilt.

The following commands can be used to verify the changes:
```bash
make clean && make -j16 && (
  cd sysroot && find . -name "*.h" -type f -exec stat -c '%n %y' {} \;
) > sysroot.v1.timestamp && \
make -j16 && (
  cd sysroot && find . -name "*.h" -type f -exec stat -c '%n %y' {} \;
) > sysroot.v2.timestamp
diff sysroot.v1.timestamp sysroot.v2.timestamp
```
The diff should show no changes in timestamps.

Makefile

@@ -764,32 +764,39 @@ $(LIBWASI_EMULATED_PROCESS_CLOCKS_OBJS) $(LIBWASI_EMULATED_PROCESS_CLOCKS_SO_OBJ
 $(EMMALLOC_OBJS): CFLAGS += \
     -fno-strict-aliasing
 
-include_dirs:
-	#
-	# Install the include files.
-	#
-	mkdir -p "$(SYSROOT_INC)"
-	cp -r "$(LIBC_BOTTOM_HALF_HEADERS_PUBLIC)"/* "$(SYSROOT_INC)"
-
+$(SYSROOT_INC)/bits/alltypes.h: $(LIBC_TOP_HALF_MUSL_DIR)/tools/mkalltypes.sed $(LIBC_TOP_HALF_MUSL_DIR)/arch/wasm32/bits/alltypes.h.in $(LIBC_TOP_HALF_MUSL_DIR)/include/alltypes.h.in
 	# Generate musl's bits/alltypes.h header.
 	mkdir -p "$(SYSROOT_INC)/bits"
 	sed -f $(LIBC_TOP_HALF_MUSL_DIR)/tools/mkalltypes.sed \
 	    $(LIBC_TOP_HALF_MUSL_DIR)/arch/wasm32/bits/alltypes.h.in \
 	    $(LIBC_TOP_HALF_MUSL_DIR)/include/alltypes.h.in \
 	    > "$(SYSROOT_INC)/bits/alltypes.h"
 
+$(SYSROOT_INC)/__wasi_snapshot.h:
+	mkdir -p "$(SYSROOT_INC)"
+ifeq ($(WASI_SNAPSHOT), p2)
+	printf '#ifndef __wasilibc_use_wasip2\n#define __wasilibc_use_wasip2\n#endif\n' \
+		> "$(SYSROOT_INC)/__wasi_snapshot.h"
+else
+	printf '/* This file is (practically) empty by default.  The Makefile will replace it\n   with a non-empty version that defines `__wasilibc_use_wasip2` if targeting\n   `wasm32-wasip2`.\n */\n\n' \
+		> "$(SYSROOT_INC)/__wasi_snapshot.h"
+endif
+
+include_dirs: $(SYSROOT_INC)/bits/alltypes.h $(SYSROOT_INC)/__wasi_snapshot.h
+	#
+	# Install the include files.
+	#
+	mkdir -p "$(SYSROOT_INC)"
+	cp -p -r "$(LIBC_BOTTOM_HALF_HEADERS_PUBLIC)"/* "$(SYSROOT_INC)"
+
 	# Copy in the bulk of musl's public header files.
-	cp -r "$(LIBC_TOP_HALF_MUSL_INC)"/* "$(SYSROOT_INC)"
+	cp -p -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"
+	cp -p -r "$(LIBC_TOP_HALF_MUSL_DIR)"/arch/generic/bits/* "$(SYSROOT_INC)/bits"
+	cp -p -r "$(LIBC_TOP_HALF_MUSL_DIR)"/arch/wasm32/bits/* "$(SYSROOT_INC)/bits"
 
 	# Remove selected header files.
 	$(RM) $(patsubst %,$(SYSROOT_INC)/%,$(MUSL_OMIT_HEADERS))
-ifeq ($(WASI_SNAPSHOT), p2)
-	printf '#ifndef __wasilibc_use_wasip2\n#define __wasilibc_use_wasip2\n#endif\n' \
-		> "$(SYSROOT_INC)/__wasi_snapshot.h"
-endif
 
 startup_files: include_dirs $(LIBC_BOTTOM_HALF_CRT_OBJS)
 	#
@@ -981,7 +988,7 @@ check-symbols: startup_files libc
 
 install: finish
 	mkdir -p "$(INSTALL_DIR)"
-	cp -r "$(SYSROOT)/lib" "$(SYSROOT)/share" "$(SYSROOT)/include" "$(INSTALL_DIR)"
+	cp -p -r "$(SYSROOT)/lib" "$(SYSROOT)/share" "$(SYSROOT)/include" "$(INSTALL_DIR)"
 
 $(BINDING_WORK_DIR)/wasi-cli:
 	mkdir -p "$(BINDING_WORK_DIR)"