Fixes install prefix.

Author: lygstate

cmake/iotjs.cmake

@@ -568,6 +568,9 @@ endif()
 if(NOT BUILD_LIB_ONLY)
   set(TARGET_IOTJS iotjs)
   message(STATUS "CMAKE_BINARY_DIR        ${CMAKE_BINARY_DIR}")
+  if (NOT INSTALL_PREFIX)
+    set(INSTALL_PREFIX ${CMAKE_BINARY_DIR}/install)
+  endif()
   message(STATUS "BINARY_INSTALL_DIR      ${INSTALL_PREFIX}/bin")
   message(STATUS "LIBRARY_INSTALL_DIR     ${INSTALL_PREFIX}/lib")