configure: Add a --disable-version-tracking option

This disables regenerating ffversion.h whenever the checked out
git commit changes, speeding up development rebuilds.

Whenever this option is set, force the version to be printed as
"unknown" rather than showing potentially stale information.

Signed-off-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Paul B Mahol <onemda@gmail.com>

Makefile

@@ -133,13 +133,18 @@ endif
 	$(LD) $(LDFLAGS) $(LDEXEFLAGS) $(LD_O) $(OBJS-$*) $(FF_EXTRALIBS)
 
 VERSION_SH  = $(SRC_PATH)/ffbuild/version.sh
+ifeq ($(VERSION_TRACKING),yes)
 GIT_LOG     = $(SRC_PATH)/.git/logs/HEAD
+endif
 
 .version: $(wildcard $(GIT_LOG)) $(VERSION_SH) ffbuild/config.mak
 .version: M=@
 
+ifneq ($(VERSION_TRACKING),yes)
+libavutil/ffversion.h .version: REVISION=unknown
+endif
 libavutil/ffversion.h .version:
-	$(M)$(VERSION_SH) $(SRC_PATH) libavutil/ffversion.h $(EXTRA_VERSION)
+	$(M)revision=$(REVISION) $(VERSION_SH) $(SRC_PATH) libavutil/ffversion.h $(EXTRA_VERSION)
 	$(Q)touch .version
 
 # force version.sh to run whenever version might have changed

configure

@@ -504,6 +504,7 @@ Developer options (useful when working on FFmpeg itself):
   --enable-macos-kperf     enable macOS kperf (private) API
   --disable-large-tests    disable tests that use a large amount of memory
   --disable-ptx-compression don't compress CUDA PTX code even when possible
+  --disable-version-tracking don't include the git/release version in the build
 
 NOTE: Object files are built at the place where configure is launched.
 EOF
@@ -2576,6 +2577,7 @@ CMDLINE_SELECT="
     optimizations
     rpath
     stripping
+    version_tracking
 "
 
 PATHS_LIST="
@@ -3992,6 +3994,7 @@ enable $DOCUMENT_LIST
 enable $EXAMPLE_LIST
 enable $LIBRARY_LIST
 enable stripping
+enable version_tracking
 
 enable asm
 enable debug
@@ -8023,6 +8026,7 @@ SAMPLES:=${samples:-\$(FATE_SAMPLES)}
 NOREDZONE_FLAGS=$noredzone_flags
 LIBFUZZER_PATH=$libfuzzer_path
 IGNORE_TESTS=$ignore_tests
+VERSION_TRACKING=$version_tracking
 EOF
 
 map 'eval echo "${v}_FFLIBS=\$${v}_deps" >> ffbuild/config.mak' $LIBRARY_LIST