Auxiliary: Add options to control Vim and Emacs file installation

Since commit 2642f432ef (Aux: Install editor and bash files to more
natural locations, 2020-03-30, v3.18.0-rc1~429^2) these files are
installed by default into locations that are not CMake-specific but may
be distro-specific.  Add options for packagers to control these
locations.

Also rename the `CMAKE_BASH_COMP_DIR` option to follow our conventions.

Fixes: #20993

Auxiliary/CMakeLists.txt

@@ -1,4 +1,16 @@
-install(DIRECTORY vim/indent vim/syntax DESTINATION ${CMAKE_XDGDATA_DIR}/vim/vimfiles)
-install(FILES cmake-mode.el DESTINATION ${CMAKE_XDGDATA_DIR}/emacs/site-lisp)
+# Install Vim files to a typical system integration directory.
+# Packagers can set CMake_INSTALL_VIMFILES_DIR to control this.
+if(NOT CMake_INSTALL_VIMFILES_DIR)
+  set(CMake_INSTALL_VIMFILES_DIR ${CMAKE_XDGDATA_DIR}/vim/vimfiles)
+endif()
+install(DIRECTORY vim/indent vim/syntax DESTINATION ${CMake_INSTALL_VIMFILES_DIR})
+
+# Install Emacs files to a typical system integration directory.
+# Packagers can set CMake_INSTALL_EMACS_DIR to control this.
+if(NOT CMake_INSTALL_EMACS_DIR)
+  set(CMake_INSTALL_EMACS_DIR ${CMAKE_XDGDATA_DIR}/emacs/site-lisp)
+endif()
+install(FILES cmake-mode.el DESTINATION ${CMake_INSTALL_EMACS_DIR})
+
 install(FILES cmake.m4 DESTINATION ${CMAKE_XDGDATA_DIR}/aclocal)
 add_subdirectory (bash-completion)

Auxiliary/bash-completion/CMakeLists.txt

@@ -6,11 +6,16 @@
 # with) as well as a simple installation by a local user into their home
 # directory *if* the prefix is `$HOME/.local` since `.local/share/` is part of
 # the bash-completion search path too.
-# For more complex installations, packagers can set CMAKE_BASH_COMP_DIR to
-# another system location.
+# For more complex installations, packagers can set CMake_INSTALL_BASH_COMP_DIR
+# to another system location.
 
-set(CMAKE_BASH_COMP_DIR_DEFAULT ${CMAKE_XDGDATA_DIR}/bash-completion/completions)
-if (NOT CMAKE_BASH_COMP_DIR)
-  set(CMAKE_BASH_COMP_DIR "${CMAKE_BASH_COMP_DIR_DEFAULT}")
+if(NOT CMake_INSTALL_BASH_COMP_DIR)
+  if(CMAKE_BASH_COMP_DIR)
+    # Honor previous customization option.
+    set(CMake_INSTALL_BASH_COMP_DIR "${CMAKE_BASH_COMP_DIR}")
+  else()
+    # Default.
+    set(CMake_INSTALL_BASH_COMP_DIR ${CMAKE_XDGDATA_DIR}/bash-completion/completions)
+  endif()
 endif()
-install(FILES cmake cpack ctest DESTINATION ${CMAKE_BASH_COMP_DIR})
+install(FILES cmake cpack ctest DESTINATION ${CMake_INSTALL_BASH_COMP_DIR})