kbuild: change kbuild to not rely on incorrect GNU make behavior

The kbuild system takes advantage of an incorrect behavior in GNU make.
Once this behavior is fixed, all files in the kernel rebuild every time,
even if nothing has changed.  This patch ensures kbuild works with both
the incorrect and correct behaviors of GNU make.

For more details on the incorrect behavior, see:

http://lists.gnu.org/archive/html/bug-make/2006-03/msg00003.html

Changes in this patch:
  - Keep all targets that are to be marked .PHONY in a variable, PHONY.
  - Add .PHONY: $(PHONY) to mark them properly.
  - Remove any $(PHONY) files from the $? list when determining whether
    targets are up-to-date or not.

Signed-off-by: Paul Smith <psmith@gnu.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

1 files changed, 9 insertions, 3 deletions

diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index 563e3c5..0cfbe1c 100644
--- a/scripts/Makefile.modpost
+++ b/scripts/Makefile.modpost
@@ -32,7 +32,7 @@
 # Step 4 is solely used to allow module versioning in external modules,
 # where the CRC of each module is retrieved from the Module.symers file.
 
-.PHONY: _modpost
+PHONY := _modpost
 _modpost: __modpost
 
 include .config
@@ -60,7 +60,7 @@ quiet_cmd_modpost = MODPOST
 	$(if $(KBUILD_EXTMOD),-o $(modulesymfile)) \
 	$(filter-out FORCE,$^)
 
-.PHONY: __modpost
+PHONY += __modpost
 __modpost: $(wildcard vmlinux) $(modules:.ko=.o) FORCE
 	$(call cmd,modpost)
 
@@ -97,7 +97,7 @@ targets += $(modules)
 # Add FORCE to the prequisites of a target to force it to be always rebuilt.
 # ---------------------------------------------------------------------------
 
-.PHONY: FORCE
+PHONY += FORCE
 
 FORCE:
 
@@ -112,3 +112,9 @@ cmd_files := $(wildcard $(foreach f,$(targets),$(dir $(f)).$(notdir $(f)).cmd))
 ifneq ($(cmd_files),)
   include $(cmd_files)
 endif
+
+
+# Declare the contents of the .PHONY variable as phony.  We keep that
+# information in a variable se we can use it in if_changed and friends.
+
+.PHONY: $(PHONY)