Add External blas definition, for morse for example

CMakeLists.txt

@@ -78,7 +78,9 @@ if( SCALFMM_USE_BLAS )
   OPTION( SCALFMM_USE_MKL_AS_BLAS "Set to ON to use MKL CBLAS" OFF )
   if( SCALFMM_USE_MKL_AS_BLAS )
      SET(BLAS_LIBRARIES "-L$ENV{MKLROOT}/lib  -lmkl_intel_lp64 -lmkl_sequential -lmkl_core")
-  else() 
+  elseif(SCALFMM_USE_EXTERNAL_BLAS) 
+     MESSAGE(STATUS "BLAS SET BY EXTERNAL PROGRAM = ${BLAS_LIBRARIES}")
+  else()
 	INCLUDE(FindBLAS)
 	INCLUDE(FindLAPACK)
   endif()