Bug in header installation (root CMakeLists.txt)

[zzxyz]

When CMAKE_INSTALL_PREFIX is relative, such as "_install", headers get installed to:
./_build/_install/_install/include

I believe the correct fix is the removing ${CMAKE_INSTALL_PREFIX} since cmake should already be handling that implicitly:
FOREACH (file ${msgpackc_HEADERS})
GET_FILENAME_COMPONENT (dir ${file} PATH)
(remove) INSTALL (FILES ${file} DESTINATION ${CMAKE_INSTALL_PREFIX}/${dir})
(add) INSTALL (FILES ${file} DESTINATION ${dir})
ENDFOREACH ()