openblas: remove {system = true} and force to use gfortran if not find openmp

[ivaquero]

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[waruqi]

we should use package:config("fortran")

[ivaquero]

Sorry, I don't get it. Do you mean package:config("gfortran")? I don't see any config options in gfortran/xmake.lua.

[waruqi]

    add_configs("fortran", {description = "Compile with fortran enabled.", default = is_plat("linux"), type = "boolean"})

openblas's configs