CMake update to avoid directxmath subdir for include (#224)

CMakeLists.txt

@@ -30,7 +30,7 @@ add_library(${PROJECT_NAME} INTERFACE)
 
 target_include_directories(${PROJECT_NAME} INTERFACE
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/Inc>
-  $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}/directxmath>)
+  $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
 
 target_compile_features(${PROJECT_NAME} INTERFACE cxx_std_11)
 
@@ -61,7 +61,7 @@ install(EXPORT ${PROJECT_NAME}-targets
   DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/${PACKAGE_NAME})
 
 install(FILES ${LIBRARY_HEADERS}
-  DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/directxmath)
+  DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
 
 install(FILES
     ${CMAKE_CURRENT_BINARY_DIR}/${PACKAGE_NAME}-config.cmake
@@ -71,7 +71,7 @@ install(FILES
 # Create pkg-config file
 include(build/JoinPaths.cmake)
 # from: https://github.com/jtojnar/cmake-snips#concatenating-paths-when-building-pkg-config-files
-join_paths(DIRECTXMATH_INCLUDEDIR_FOR_PKG_CONFIG "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}/directxmath")
+join_paths(DIRECTXMATH_INCLUDEDIR_FOR_PKG_CONFIG "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
 join_paths(DIRECTXMATH_LIBDIR_FOR_PKG_CONFIG "\${prefix}"     "${CMAKE_INSTALL_LIBDIR}")
 
 configure_file(