diff --git a/external/external_build.cmake b/external/external_build.cmake
index 029f429..9fd7a81 100644
--- a/external/external_build.cmake
+++ b/external/external_build.cmake
@@ -14,12 +14,12 @@ ENDIF(ENABLE_OPENMP)
 SET(BUILD_PREFIX ${CMAKE_BINARY_DIR}/ep_build)
 
 SET(INTERNAL_FFTW OFF)
-SET(INTERNAL_GSL ON)
-SET(INTERNAL_BOOST ON)
-SET(INTERNAL_NETCDF ON)
-SET(INTERNAL_HDF5 ON)
-SET(INTERNAL_GENGETOPT ON)
-SET(INTERNAL_QHULL ON)
+OPTION(INTERNAL_GSL "Use internal GSL" ON)
+OPTION(INTERNAL_BOOST "Use internal Boost" ON)
+OPTION(INTERNAL_NETCDF "Use internal netcdf" ON)
+OPTION(INTERNAL_HDF5 "Use internal HDF5" ON)
+OPTION(INTERNAL_GENGETOPT "Use internal gengetopt" ON)
+OPTION(INTERNAL_QHULL "Use internal qhull" ON)
 
 IF(INTERNAL_GENGETOPT)
   SET(GENGETOPT_URL "ftp://ftp.gnu.org/gnu/gengetopt/gengetopt-2.22.5.tar.gz" CACHE STRING "URL to download gengetopt from")