Need also to install the internal headers

Author: zonca

CMakeLists.txt

@@ -65,9 +65,11 @@ SET_TARGET_PROPERTIES(${SHARED_EXAMPLE_TARGET}
 TARGET_LINK_LIBRARIES(${SHARED_EXAMPLE_TARGET} OpenMM)
 INSTALL_TARGETS(/lib RUNTIME_DIRECTORY /lib ${SHARED_EXAMPLE_TARGET})
 
-# install headers excluding headers in internal
+# install headers
 FILE(GLOB API_ONLY_INCLUDE_FILES "openmmapi/include/*.h")
 INSTALL (FILES ${API_ONLY_INCLUDE_FILES} DESTINATION include)
+FILE(GLOB API_ONLY_INCLUDE_FILES_INTERNAL "openmmapi/include/internal/*.h")
+INSTALL (FILES ${API_ONLY_INCLUDE_FILES_INTERNAL} DESTINATION include/internal)
 
 # Enable testing