Avoid build-time race condition when cross-compiling from a source tarball

Since commit 66529eef88 ("Force to update revision.h when commits
differ"), the Makefile rule for $(REVISION_H) is always marked as PHONY
when building from a source tarball. This has no benefit,
file2lastrev.rb will just print a warning and exit if we're not in a VCS
checkout, but it can cause problems.

When cross-compiling, the generated file $(arch)-fake.rb is used by
every invocation of MINIRUBY. $(arch)-fake.rb depends on $(REVISION_H),
so it is considered perpetually out-of-date and sub-make invocations
will repeatedly regenerate this file.

There is no enforcement of dependencies between sub-make invocations, so
one sub-make can be running MINIRUBY (and so using $(arch)-fake.rb) at
the same time as a different sub-make is regenerating $(arch)-fake.rb.
Truncating and re-writing $(arch)-fake.rb while it is mmapped by a
running ruby interpreter can cause a SIGBUS error, for example:

  i686-linux-gnu-fake.rb: [BUG] Bus Error at 0x00007fee11f02765

No addition of dependencies to Makefile rules can address this. Instead,
we need to stop treating $(REVISION_H) as PHONY when not building from a
git checkout.

Signed-off-by: Paul Barker <paul@pbarker.dev>

defs/gmake.mk

@@ -436,9 +436,6 @@ endif
 
 ifeq ($(HAVE_GIT),yes)
 REVISION_LATEST := $(shell $(GIT_IN_SRC) rev-parse HEAD 2>/dev/null)
-else
-REVISION_LATEST := update
-endif
 REVISION_IN_HEADER := $(shell sed '/^\#define RUBY_FULL_REVISION "\(.*\)"/!d;s//\1/;q' $(wildcard $(srcdir)/revision.h revision.h) /dev/null 2>/dev/null)
 ifeq ($(REVISION_IN_HEADER),)
 REVISION_IN_HEADER := none
@@ -446,6 +443,7 @@ endif
 ifneq ($(REVISION_IN_HEADER),$(REVISION_LATEST))
 $(REVISION_H): PHONY
 endif
+endif
 
 include $(top_srcdir)/yjit/yjit.mk
 include $(top_srcdir)/zjit/zjit.mk