[metapackage] stdlib: enable external BLAS/LAPACK

[perazz]

Enable building fpm packages with stdlib linked to an external BLAS/LAPACK library

The stdlib metapackage is configure to use the external BLAS/LAPACK implementation when both blas and stdlib are enabled.

[dependencies]
stdlib = "*"
blas = "*"

The external preprocessor macros are accordingly defined:
STDLIB_EXTERNAL_BLAS
STDLIB_EXTERNAL_LAPACK

These macros allow stdlib to disable internal fallbacks and delegate linear algebra routines to the external library.

A test/example package is provided in:

example_packages/metapackage_stdlib_ext_blas