CMake: workaround unsupported cmake_path(IS_PREFIX ..) in v3.20

Add a macro that implements something similar to cmake_path's IS_PREFIX
which isn't supported in older versions of CMake.

This caused the build-bot to fail.

build_files/cmake/macros.cmake

@@ -57,6 +57,25 @@ macro(path_ensure_trailing_slash
   unset(_path_sep)
 endmacro()
 
+# Our own version of `cmake_path(IS_PREFIX ..)`.
+# This can be removed when 3.20 or greater is the minimum supported version.
+macro(path_is_prefix
+  path_prefix path result_var
+  )
+  # Remove when CMAKE version is bumped to "3.20" or greater.
+  # `cmake_path(IS_PREFIX ${path_prefix} ${path} NORMALIZE result_var)`
+  # Get the normalized paths (needed to remove `..`).
+  get_filename_component(_abs_prefix "${${path_prefix}}" ABSOLUTE)
+  get_filename_component(_abs_suffix "${${path}}" ABSOLUTE)
+  string(LENGTH "${_abs_prefix}" _len)
+  string(SUBSTRING "${_abs_suffix}" 0 "${_len}" _substr)
+  string(COMPARE EQUAL "${_abs_prefix}" "${_substr}" "${result_var}")
+  unset(_abs_prefix)
+  unset(_abs_suffix)
+  unset(_len)
+  unset(_substr)
+endmacro()
+
 # foo_bar.spam --> foo_barMySuffix.spam
 macro(file_suffix
   file_name_new file_name file_suffix

build_files/cmake/platform/platform_unix.cmake

@@ -151,7 +151,7 @@ if(WITH_PYTHON)
     # Installing into `site-packages`, warn when installing into `./../lib/`
     # which script authors almost certainly don't want.
     if(EXISTS ${LIBDIR})
-      cmake_path(IS_PREFIX LIBDIR "${PYTHON_SITE_PACKAGES}" NORMALIZE _is_prefix)
+      path_is_prefix(LIBDIR PYTHON_SITE_PACKAGES _is_prefix)
       if(_is_prefix)
         message(WARNING "
 Building Blender with the following configuration:

tests/CMakeLists.txt

@@ -36,7 +36,7 @@ set(TEST_PYTHON_EXE_EXTRA_ARGS)
 
 # Check if this a Blender managed Python installation, if so, don't add `*.pyc` files.
 if(LIBDIR)
-  cmake_path(IS_PREFIX LIBDIR "${TEST_PYTHON_EXE}" NORMALIZE _is_prefix)
+  path_is_prefix(LIBDIR TEST_PYTHON_EXE _is_prefix)
   if(_is_prefix)
     # Keep the Python in Blender's SVN LIBDIR pristine, to avoid conflicts on updating.
     set(TEST_PYTHON_EXE_EXTRA_ARGS "-B")