[CMake] Do not require BLAS

Don't end configure if BLAS is not found. Only the SuperLU
test should fail, which will be the case anyway.

(cherry picked from commit 4e461be4)
Signed-off-by: Steffen Müthing <muething@dune-project.org>

cmake/modules/FindSuperLU.cmake

@@ -16,7 +16,7 @@
 #
 
 # look for BLAS
-find_package(BLAS QUIET REQUIRED)
+find_package(BLAS QUIET)
 if(NOT BLAS_FOUND)
   message(WARNING "SuperLU requires BLAS which was not found, skipping the test.")
   return()