Properly persist git hash on each build (#1391)

Properly persist git hash on each build

Rebuild `git_hash.h` only when `.git` folder changes

`add_custom_target()` always considers its target out-of-date which
leads to rebuilding of `git_hash.h` on every build (which is
ironically what we wanted) and consequently rebuilding of
build_options.c and relinking of c3c even when no changes are made,
which is mildly annoying.

We are replacing `add_custom_target()` with `add_custom_command()`
which depends on `.git`, so `git_hash.h` is only rebuilt if any git
commands are performed. Which is less annoying.

In case of no `.git` we simply do not depend on it which leads to
`git_hash.h` being rebuilt only once.

git_hash.cmake

@@ -0,0 +1,14 @@
+find_package(Git QUIET)
+
+set(GIT_HASH "unknown")
+
+if(GIT_FOUND AND EXISTS "${CMAKE_CURRENT_LIST_DIR}/.git")
+    execute_process(COMMAND ${GIT_EXECUTABLE} rev-parse HEAD
+                    OUTPUT_VARIABLE GIT_HASH
+                    OUTPUT_STRIP_TRAILING_WHITESPACE
+                    COMMAND_ERROR_IS_FATAL ANY)
+endif()
+
+message("Git Hash: ${GIT_HASH}")
+
+file(WRITE ${CMAKE_BINARY_DIR}/git_hash.h "#pragma once\n#define GIT_HASH \"${GIT_HASH}\"\n")