diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2690baf..565a80e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -54,7 +54,7 @@ IF(YORICK_SUPPORT)
   ENDIF((EXISTS ${NETCDFCPP_INCLUDE_PATH}/netcdf AND ${NETCDFCPP_LIBRARY} MATCHES "netcdf_c\\+\\+4") OR (INTERNAL_NETCDF))
 ENDIF(YORICK_SUPPORT)
 
-find_program(CYTHON cython3 cython)
+find_program(CYTHON NAMES cython3 cython)
 find_library(ZLIB_LIBRARY z)
 find_library(DL_LIBRARY dl)
 find_library(MATH_LIBRARY m)
diff --git a/external/external_build.cmake b/external/external_build.cmake
index bd4b18b..5459427 100644
--- a/external/external_build.cmake
+++ b/external/external_build.cmake
@@ -2,8 +2,8 @@ include(FindOpenMP)
 
 OPTION(ENABLE_OPENMP "Set to Yes if Healpix and/or you need openMP" OFF)
 
-SET(FFTW_URL "http://www.fftw.org/fftw-3.3.3.tar.gz" CACHE URL "URL to download FFTW from")
-SET(EIGEN_URL "https://gitlab.com/libeigen/eigen/-/archive/3.3.7/eigen-3.3.7.tar.bz2" CACHE URL "URL to download Eigen from")
+SET(FFTW_URL "http://www.fftw.org/fftw-3.3.3.tar.gz" CACHE STRING "URL to download FFTW from")
+SET(EIGEN_URL "https://gitlab.com/libeigen/eigen/-/archive/3.3.7/eigen-3.3.7.tar.bz2" CACHE STRING "URL to download Eigen from")
 SET(GENGETOPT_URL "ftp://ftp.gnu.org/gnu/gengetopt/gengetopt-2.22.5.tar.gz" CACHE STRING "URL to download gengetopt from")
 SET(HDF5_URL "https://support.hdfgroup.org/ftp/HDF5/releases/hdf5-1.8/hdf5-1.8.18/src/hdf5-1.8.18.tar.bz2" CACHE STRING "URL to download HDF5 from")
 SET(NETCDF_URL "ftp://ftp.unidata.ucar.edu/pub/netcdf/netcdf-4.5.0.tar.gz" CACHE STRING "URL to download NetCDF from")
@@ -291,13 +291,19 @@ IF(INTERNAL_GSL)
   set(GSL_LIBRARIES ${GSL_LIBRARY} ${GSLCBLAS_LIBRARY})
   SET(cosmotool_DEPS ${cosmotool_DEPS} gsl)
 ELSE(INTERNAL_GSL)
-  find_path(GSL_INCLUDE_PATH NAMES gsl/gsl_blas.h)
-  find_library(GSL_LIBRARY gsl)
-  find_library(GSLCBLAS_LIBRARY gslcblas)
+  IF (NOT DEFINED GSL_LIBRARY OR NOT GSL_LIBRARY)
+    find_library(GSL_LIBRARY gsl)
+  ENDIF()
+  IF (NOT DEFINED GSL_INCLUDE_PATH OR NOT GSL_INCLUDE_PATH)
+    find_path(GSL_INCLUDE_PATH NAMES gsl/gsl_blas.h)
+  ENDIF()
+  IF (NOT DEFINED GSL_CBLAS_LIBRARY OR NOT GSL_CBLAS_LIBRARY)
+    find_library(GSLCBLAS_LIBRARY gslcblas)
+  ENDIF()
 
   set(GSL_LIBRARIES ${GSL_LIBRARY} ${GSLCBLAS_LIBRARY})
-
 ENDIF(INTERNAL_GSL)
+
 mark_as_advanced(GSL_LIBRARY GSLCBLAS_LIBRARY GSL_INCLUDE_PATH)