Prefer arch-independed paths for install

xsimd is header only library, so prefer arch-independed paths for cmake and
pkg-config.

References:
 * xtensor-stack/xtl@d877d94
 * xtensor-stack/xtensor@7738389

CMakeLists.txt

@@ -139,7 +139,7 @@ install(DIRECTORY ${XSIMD_INCLUDE_DIR}/xsimd
         DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 
 # GNUInstallDirs "DATADIR" wrong here; CMake search path wants "share".
-set(XSIMD_CMAKECONFIG_INSTALL_DIR "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}" CACHE STRING "install path for xsimdConfig.cmake")
+set(XSIMD_CMAKECONFIG_INSTALL_DIR "${CMAKE_INSTALL_DATADIR}/cmake/${PROJECT_NAME}" CACHE STRING "install path for xsimdConfig.cmake")
 
 configure_package_config_file(${PROJECT_NAME}Config.cmake.in
                               "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}Config.cmake"
@@ -164,4 +164,4 @@ configure_file(${PROJECT_NAME}.pc.in
                "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"
                 @ONLY)
 install(FILES "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"
-        DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig/")
+        DESTINATION "${CMAKE_INSTALL_DATADIR}/pkgconfig/")