feat: install cmake config module in arch-indep location

toml11 is an header-only library, and the exported cmake target for it
only points to the headers; because of this, it seems a good idea to
install the cmake config module in an architecture-independent location
for cmake, rather than under a library (architecture-dependent)
location.

Doing so is easy:
- tweak TOML11_INSTALL_CMAKE_DIR to point to a cmake subdirectory of the
  shared dataroot location (typically $prefix/share)
- pass ARCH_INDEPENDENT argument to write_basic_package_version_file():
  this is because the cmake config module generated by default contains
  the architecture bits check (32bit vs 64bit vs ...); since it does
  not matter in this case, make the config module arch-independent

CMakeLists.txt

@@ -56,7 +56,7 @@ check_cxx_compiler_flag("-Wundef"               TOML11_COMPILER_SUPPORTS_WUNDEF)
 check_cxx_compiler_flag("-Wshadow"              TOML11_COMPILER_SUPPORTS_WSHADOW)
 
 include(GNUInstallDirs)
-set(TOML11_INSTALL_CMAKE_DIR   ${CMAKE_INSTALL_LIBDIR}/cmake/toml11)
+set(TOML11_INSTALL_CMAKE_DIR   ${CMAKE_INSTALL_DATAROOTDIR}/cmake/toml11)
 set(TOML11_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR})
 set(TOML11_CONFIG_DIR          ${CMAKE_CURRENT_BINARY_DIR}/cmake)
 set(TOML11_CONFIG              ${TOML11_CONFIG_DIR}/toml11Config.cmake)

src/CMakeLists.txt

@@ -153,6 +153,7 @@ if(TOML11_INSTALL)
     write_basic_package_version_file(${TOML11_CONFIG_VERSION}
         VERSION ${toml11_VERSION}
         COMPATIBILITY SameMajorVersion
+        ARCH_INDEPENDENT
         )
 
     configure_package_config_file(