Fix parallel build of gnu-efi

When building with make 4.4.1, gnu-efi may be built twice and in parallel:

  $ make -n -j ARCH=x86_64 2>&1 | tee build.log
  $ grep -e 'make -C gnu-efi' build.log
  make -C gnu-efi \
  make -C gnu-efi \

This has been seen to cause linking failures when building libgnuefi.a because
some object files may end up truncated.

The reason for this is that make interprets multiple targets in the same rule
as independent and can run the rule multiple times. The solution is to define a
grouped target with &:, which causes make to behave the way one expects: runs
the rule once and expects it to create all targets.

Grouped target support was added in make 4.3 released 4 years ago.

Signed-off-by: Jeremi Piotrowski <[email protected]>

Makefile

@@ -154,7 +154,7 @@ $(MMSONAME): $(MOK_OBJS) $(LIBS)
 	$(LD) -o $@ $(LDFLAGS) $^ $(EFI_LIBS) lib/lib.a
 
 gnu-efi/$(ARCH_GNUEFI)/gnuefi/libgnuefi.a gnu-efi/$(ARCH_GNUEFI)/lib/libefi.a: CFLAGS+=-DGNU_EFI_USE_EXTERNAL_STDARG
-gnu-efi/$(ARCH_GNUEFI)/gnuefi/libgnuefi.a gnu-efi/$(ARCH_GNUEFI)/lib/libefi.a:
+gnu-efi/$(ARCH_GNUEFI)/gnuefi/libgnuefi.a gnu-efi/$(ARCH_GNUEFI)/lib/libefi.a &:
 	mkdir -p gnu-efi/lib gnu-efi/gnuefi
 	$(MAKE) -C gnu-efi \
 		COMPILER="$(COMPILER)" \