cmake: Respect --prefix for headers during cmake --install

Removing CMAKE_INSTALL_PREFIX allows for `cmake --install <build-dir>
--prefix=<installation-prefix>` to install headers at
`<installation-prefix>/include` at install time.

When CMAKE_INSTALL_PREFIX is present in `INSTALL()`, it is hard-coded
during configuration (e.g., `cmake -S .`), which disallows it from being
set at install time.

Author: ericvw

CMakeLists.txt

@@ -260,11 +260,11 @@ INSTALL (TARGETS ${MSGPACK_INSTALLTARGETS} EXPORT msgpack-c-targets
 )
 FOREACH (file ${msgpack-c_common_HEADERS})
     GET_FILENAME_COMPONENT (dir ${file} PATH)
-    INSTALL (FILES ${file} DESTINATION ${CMAKE_INSTALL_PREFIX}/${dir})
+    INSTALL (FILES ${file} DESTINATION ${dir})
 ENDFOREACH ()
 FOREACH (file ${msgpack-c_configured_HEADERS})
     GET_FILENAME_COMPONENT (dir ${file} PATH)
-    INSTALL (FILES ${CMAKE_CURRENT_BINARY_DIR}/${file} DESTINATION ${CMAKE_INSTALL_PREFIX}/${dir})
+    INSTALL (FILES ${CMAKE_CURRENT_BINARY_DIR}/${file} DESTINATION ${dir})
 ENDFOREACH ()
 IF (NOT MSVC)
     INSTALL (FILES ${CMAKE_CURRENT_BINARY_DIR}/msgpack-c.pc DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig)