Fix version detection for out-of-source builds

Git was called in the build directory and not the source directory.
This works fine when the build directory resides within the source
repository, but doesn't work for out-of-source builds. Example:

```
~/zigbuild$ cmake ../zig
fatal: not a git repository (or any of the parent directories): .git
Configuring zig version 0.2.0+
```

Use Git's `-C <path>` flag to always point to the source directory so
that it doesn't matter where the build directory lives.

CMakeLists.txt

@@ -22,7 +22,7 @@ set(ZIG_VERSION "${ZIG_VERSION_MAJOR}.${ZIG_VERSION_MINOR}.${ZIG_VERSION_PATCH}"
 find_program(GIT_EXE NAMES git)
 if(GIT_EXE)
     execute_process(
-        COMMAND ${GIT_EXE} name-rev HEAD --tags --name-only --no-undefined --always
+        COMMAND ${GIT_EXE} -C ${CMAKE_SOURCE_DIR} name-rev HEAD --tags --name-only --no-undefined --always
         OUTPUT_VARIABLE ZIG_GIT_REV
         OUTPUT_STRIP_TRAILING_WHITESPACE)
     if(ZIG_GIT_REV MATCHES "\\^0$")