fix GSL cmake config

CMakeLists.txt

@@ -205,7 +205,7 @@ endif()
 ## GSL
 find_package(GSL)
 
-if (GSL)
+if (GSL_FOUND)
     include_directories(${GSL_INCLUDE_DIRS})
     list(APPEND TURTLE_LIBS "${GSL_LIBRARIES}")
 endif()
@@ -419,7 +419,7 @@ set(hpp_for_lib
     ${PROJECT_SOURCE_DIR}/cpp/full_code/ornstein_uhlenbeck_process.hpp
     ${PROJECT_SOURCE_DIR}/cpp/full_code/ou_vorticity_equation.hpp)
 
-if(GSL)
+if(GSL_FOUND)
     LIST(APPEND cpp_for_lib ${PROJECT_SOURCE_DIR}/cpp/particles/rhs/deformation_tensor_rhs.cpp)
     LIST(APPEND hpp_for_lib ${PROJECT_SOURCE_DIR}/cpp/particles/rhs/deformation_tensor_rhs.hpp)
 endif()