cmake: Add ability to pass in GGML_BUILD_NUMBER (#1096)

This makes git as a dependency optional, and is useful in the case where
ggml is built not from git, but from a tarball, or a distribution source
package.

This conditional also affects GGML_BUILD_COMMIT. Nothing seems to be
using it, though, so there doesn't seem much value factor it out, or
even require it.

CMakeLists.txt

@@ -274,22 +274,25 @@ endif()
 
 # Generate version info based on git commit.
 
-find_program(GIT_EXE NAMES git git.exe REQUIRED NO_CMAKE_FIND_ROOT_PATH)
-execute_process(COMMAND ${GIT_EXE} rev-list --count HEAD
-    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
-    OUTPUT_VARIABLE GGML_BUILD_NUMBER
-    OUTPUT_STRIP_TRAILING_WHITESPACE
-)
-
-if(GGML_BUILD_NUMBER EQUAL 1)
-    message(WARNING "GGML build version fixed at 1 likely due to a shallow clone.")
+if(NOT DEFINED GGML_BUILD_NUMBER)
+    find_program(GIT_EXE NAMES git git.exe REQUIRED NO_CMAKE_FIND_ROOT_PATH)
+    execute_process(COMMAND ${GIT_EXE} rev-list --count HEAD
+        WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+        OUTPUT_VARIABLE GGML_BUILD_NUMBER
+        OUTPUT_STRIP_TRAILING_WHITESPACE
+    )
+
+    if(GGML_BUILD_NUMBER EQUAL 1)
+        message(WARNING "GGML build version fixed at 1 likely due to a shallow clone.")
+    endif()
+
+    execute_process(COMMAND ${GIT_EXE} rev-parse --short HEAD
+        WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+        OUTPUT_VARIABLE GGML_BUILD_COMMIT
+        OUTPUT_STRIP_TRAILING_WHITESPACE
+    )
 endif()
 
-execute_process(COMMAND ${GIT_EXE} rev-parse --short HEAD
-    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
-    OUTPUT_VARIABLE GGML_BUILD_COMMIT
-    OUTPUT_STRIP_TRAILING_WHITESPACE
-)
 
 # Capture variables prefixed with GGML_.