* Removed other checkings of empty blas.ldflags in blas.py
and test_blas.py.

* Correction in alt_gemm_template.c to prevent some test
failures in test_blas.py (occuring after above modification):
in some tests, a zero-content matrix is passed as C
(with M*N == 0). Now if we encounter this case, we just
skip it (gemm not calculated, C not modified).

* Correction in blas_headers.py to prevent some old string-
formating errors when strings are C code containing "%" symbols.

Now test_blas.py runs perfectly. I have to re-run all other
tests tonight.