[klibc] add more PHONY targets to $(PHONY)

make-3.81 excludes PHONY dependencies from $?, make 3.82+ includes them.
This leads to always rebuilding klibc targets that depend on .PHONY
dependencies.  From the make 3.82 release notes
https://lists.gnu.org/archive/html/info-gnu/2010-07/msg00023.html:
    * WARNING: Backward-incompatibility!
      The '$?' variable now contains all prerequisites that caused the target to
      be considered out of date, even if they do not exist (previously only
      existing targets were provided in $?).

Linux fixed this with commit 4f1933620f57 ("kbuild: change kbuild to not
rely on incorrect GNU make behavior").

Similar to Linux, klibc if_changed already excludes $(PHONY) from $?
when determining if a target is up-to-date.  Klibc also has a $(PHONY)
list of phony targets.  But klibc does not add many .PHONY targets to
$(PHONY).

Changes in this patch:
- add previously defined .PHONY targets to PHONY, so existing if_changed
  filtering applies to them as well
- declare $(PHONY) targets as .PHONY

Signed-off-by: Greg Thelen <gthelen@google.com>
Link: https://www.zytor.com/pipermail/klibc/2018-June/003995.html
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
diff --git a/scripts/Kbuild.install b/scripts/Kbuild.install
index 8af5697..bafd423 100644
--- a/scripts/Kbuild.install
+++ b/scripts/Kbuild.install
@@ -14,7 +14,7 @@
 SHLIBDIR = /lib
 
 # First rule
-.PHONY: __install install-rule
+PHONY := __install install-rule
 __install:
 
 # Install commands
@@ -64,7 +64,7 @@
 endif
 
 # Descending
-.PHONY: $(subdir-)
+PHONY += $(subdir-)
 $(subdir-):
 	$(Q)$(MAKE) KLIBC_INSTALL=1 \
 	            -f $(srctree)/scripts/Kbuild.install obj=$@
@@ -84,7 +84,7 @@
 # 1) Create directories, install headers and man pages
 # 2) Tell that we now install binaries
 # 3) Install binaries by descending
-.PHONY: header footer descend
+PHONY += header footer descend
 header:
 	$(Q)echo "  INSTALL headers + man pages to $(INSTALLROOT)$(INSTALLDIR)"
 	$(Q)mkdir -p $(INSTALLROOT)$(bindir)
@@ -111,3 +111,7 @@
 __install: descend
 	@:
 endif
+
+# Declare the contents of the PHONY variable as phony.  We keep the variable for
+# if_changed.
+.PHONY: $(PHONY)
diff --git a/scripts/Kbuild.klibc b/scripts/Kbuild.klibc
index f500d53..f147a37 100644
--- a/scripts/Kbuild.klibc
+++ b/scripts/Kbuild.klibc
@@ -54,7 +54,7 @@
 # Preset target and make sure it is a ':=' variable
 targets :=
 
-.phony: __build
+PHONY := __build
 __build:
 
 # Read .config if it exist, otherwise ignore
@@ -374,7 +374,7 @@
 # Descending
 # ---------------------------------------------------------------------------
 
-.PHONY: $(subdir-y) $(kprog-dirs) $(klib-dirs)
+PHONY += $(subdir-y) $(kprog-dirs) $(klib-dirs)
 $(sort $(subdir-y) $(kprog-dirs) $(klib-dirs)): $(lib-target)
 	$(Q)$(MAKE) $(klibc)=$@
 
@@ -419,3 +419,7 @@
 # Usage:
 # $(Q)$(MAKE) $(klibc)=dir
 klibc := -rR -f $(srctree)/scripts/Kbuild.klibc obj
+
+# Declare the contents of the PHONY variable as phony.  We keep the variable for
+# if_changed.
+.PHONY: $(PHONY)
diff --git a/scripts/Makefile.clean b/scripts/Makefile.clean
index a588749..abc4e3f 100644
--- a/scripts/Makefile.clean
+++ b/scripts/Makefile.clean
@@ -4,7 +4,7 @@
 
 src := $(obj)
 
-.PHONY: __clean
+.PHONY := __clean
 __clean:
 
 # Shorthand for $(Q)$(MAKE) scripts/Makefile.clean obj=dir
@@ -83,10 +83,14 @@
 # Descending
 # ---------------------------------------------------------------------------
 
-.PHONY: $(subdirs)
+PHONY += $(subdirs)
 $(subdirs):
 	$(Q)$(MAKE) $(clean)=$@
 
 # If quiet is set, only print short version of command
 
 cmd = @$(if $($(quiet)cmd_$(1)),echo '  $($(quiet)cmd_$(1))' &&) $(cmd_$(1))
+
+# Declare the contents of the PHONY variable as phony.  We keep the variable for
+# if_changed.
+.PHONY: $(PHONY)