FindSuperLU: make sure that the "SUPERLU_FOUND" variable is always set

I ran into problems (syntax errors in if statements) if that module
bailed out before find_package_handle_standard_args() was
invoked. Since OPM/ewoms#54 this happened e.g. on systems where no
BLAS library is installed.

cmake/Modules/FindSuperLU.cmake

@@ -21,6 +21,8 @@ include(CheckCSourceCompiles)
 
 cmake_push_check_state()
 
+set(SUPERLU_FOUND "FALSE")
+
 # find out the size of a pointer. this is required to only search for
 # libraries in the directories relevant for the architecture
 if (CMAKE_SIZEOF_VOID_P)