Publish header with toolchain versions

This change is an attempt to resolve [wasi-libc#490] but at the wasi-sdk
level. It adds a `version.h` header file to
`share/wasi-sysroot/include/<target>/wasi` so that users have
programmatic access to some extra information to output better error
messages, e.g. This `version.h` looks something like:

  ```c
  // Generated by wasi-sdk's `version.py` script.
  #ifndef VERSION_H
  #define VERSION_H

  #define WASI_SDK_VERSION "24.6g754aec3d6f58+m"
  #define WASI_LIBC_VERSION "b9ef79d7dbd4"

  #endif
  ```

It _is_ a bit strange that we're adding to wasi-libc's include files
from wasi-sdk (it would be cleaner if it happened in wasi-libc directly)
but wasi-sdk actually has the information available.

[wasi-libc#490]: WebAssembly/wasi-libc#490

cmake/wasi-sdk-sysroot.cmake

@@ -336,6 +336,19 @@ file(GENERATE OUTPUT ${version_file_tmp} CONTENT ${version_dump})
 add_custom_target(version-file DEPENDS ${version_file_tmp})
 add_dependencies(build version-file)
 
+# Install a `version.h` script in each sysroot's include directory.
+execute_process(
+  COMMAND ${PYTHON} ${version_script} header
+  WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+  OUTPUT_VARIABLE version_header)
+foreach(target IN LISTS WASI_SDK_TARGETS)
+  message(STATUS "Creating version.h in ${version_dir}")
+  set(version_header_path ${wasi_sysroot}/include/${target}/wasi/version.h)
+  file(GENERATE OUTPUT ${version_header_path} CONTENT ${version_header})
+  add_custom_target(version-header-${target} DEPENDS ${version_header_path})
+  add_dependencies(build version-header-${target})
+endforeach()
+
 if(WASI_SDK_INCLUDE_TESTS)
   add_subdirectory(tests)
 endif()

version.py

@@ -62,7 +62,7 @@ def parse_git_version(version):
 def git_version():
     version = exec(['git', 'describe', '--long', '--candidates=999',
                     '--match=wasi-sdk-*', '--dirty=+m', f'--abbrev={GIT_REF_LEN}'],
-                    os.path.dirname(sys.argv[0]))
+                   os.path.dirname(sys.argv[0]))
     major, minor, git, dirty = parse_git_version(version)
     version = f'{major}.{minor}'
     if git:
@@ -109,13 +109,23 @@ def main(action, llvm_dir):
         major, minor, path = llvm_cmake_version(llvm_dir)
         print(f'llvm-version: {major}.{minor}.{path}')
         print(f'config: {git_commit("src/config")}')
+    elif action == 'header':
+        print('// Generated by wasi-sdk\'s `version.py` script.')
+        print('#ifndef VERSION_H')
+        print('#define VERSION_H')
+        print()
+        print(f'#define WASI_SDK_VERSION "{git_version()}"')
+        print(f'#define WASI_LIBC_VERSION "{git_commit("src/wasi-libc")}"')
+        print()
+        print('#endif')
 
 
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(
         description='Print the various kinds of versions in wasi-sdk')
     parser.add_argument('action',
-                        choices=['wasi-sdk', 'llvm', 'llvm-major', 'dump'],
+                        choices=['wasi-sdk', 'llvm',
+                                 'llvm-major', 'dump', 'header'],
                         nargs='?',
                         default='wasi-sdk',
                         help='Which kind of version to print (default: wasi-sdk).')