Align RubyGems override to PR - ruby/rubygems#8753

Author: cfisoi

lib/rubygems/cmake_builder.rb

@@ -21,13 +21,13 @@
 #
 # Useful options to know are:
 #
-#  -G to specicy a generator (-G Ninja is recommended)
+#  -G to specify a generator (-G Ninja is recommended)
 #  -D<CMAKE_VARIABLE> to set a CMake variable (for example -DCMAKE_BUILD_TYPE=Release)
 #  --preset <preset_name> to use a preset
 #
 # If the Gem author provides presets, via CMakePresets.json file, you will likely want to us use one of them.
-# If not, then you may wish to specify a generator (Ninja is recommened since it can builds projects in parallel
-# versus a Make generator which is much slower).
+# If not, then you may wish to specify a generator (Ninja is recommended since it can build projects in parallel
+# versus a Make generator which build ins serial and thus is *much* slower).
 
 require "fileutils"
 require 'open3'
@@ -47,6 +47,9 @@ def build(extension, gem_dir, results, args = [], lib_dir = nil, cmake_dir = Dir
       warn "--target-rbconfig is not yet supported for CMake extensions. Ignoring"
     end
 
+    # Make sure lib dir is set
+    lib_dir ||= File.join(gem_dir, "lib")
+
     # Figure the build dir
     build_dir = File.join(cmake_dir, "build")
 
@@ -97,7 +100,7 @@ def check_presets(cmake_dir, args)
       stdout, stderr, status = Open3.capture3(build_env, *cmd, chdir: cmake_dir)
       message = <<~EOS
         The gem author provided a list of presets that can be used to build the gem. To use a preset specify it on the command line:
-          
+
           gem install <gem_name> -- --preset <preset_name>
 
         #{stdout}