sc_blas.m4: fix false negative in blas config
sc_blas.m4: fix false negative in blas config

SC_BLAS_LINK does not set sc_blas_ok=yes on success, we have to set that first
and then let it set sc_blas_ok=no if it fails.

johannrudi/libsc,cburstedde/libsc,cburstedde/libsc,johannrudi/libsc,holke/libsc,johannrudi/libsc,holke/libsc,cburstedde/libsc,holke/libsc,johannrudi/libsc,cburstedde/libsc,johannrudi/libsc,holke/libsc,cburstedde/libsc