keg: explicitly create cmake dir under lib

Packages supporting CMake may install configuration files for use
with CMake's find_package command. A recommended location for these
is `<prefix>/lib/cmake` which is not unique across packages. This
may cause issues for Formula using this location when their Keg
is linked.

As with pkg-config, explicitly create the `lib/cmake` folder when
linking a Keg that has installed folders/files to this location.

Also add testcase.

Closes Homebrew/homebrew#38005.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/keg.rb

@@ -278,6 +278,8 @@ class Keg
       when 'charset.alias' then :skip_file
       # pkg-config database gets explicitly created
       when 'pkgconfig' then :mkpath
+      # cmake database gets explicitly created
+      when 'cmake' then :mkpath
       # lib/language folders also get explicitly created
       when 'dtrace' then :mkpath
       when /^gdk-pixbuf/ then :mkpath

Library/Homebrew/test/test_keg.rb

@@ -207,6 +207,13 @@ class LinkTests < Homebrew::TestCase
     assert_predicate link.lstat, :directory?
   end
 
+  def test_cmake_is_mkpathed
+    link = HOMEBREW_PREFIX.join("lib", "cmake")
+    @keg.join("lib", "cmake").mkpath
+    @keg.link
+    assert_predicate link.lstat, :directory?
+  end
+
   def test_symlinks_are_linked_directly
     link = HOMEBREW_PREFIX.join("lib", "pkgconfig")