[WIP] Enhance MTLN performance

[lmdiazangulo]

Profiling seems to show that the problem is with ngspice. A possible solution would be to keep calls to it at minimum. Just when the input contains "circuits" which require it.

[Alberto-o]

@copilot ubuntu intel compiler and windows intel compiler fail during build due to a problem with the fortran intrinsic routine symlnk. The ubuntu gcc compiler gives no problema with this routine. Explain why and propose a solution

[Copilot]

@copilot ubuntu intel compiler and windows intel compiler fail during build due to a problem with the fortran intrinsic routine symlnk. The ubuntu gcc compiler gives no problema with this routine. Exp...

Why SYMLNK fails on Intel compilers:

SYMLNK is a GFortran-specific non-standard extension — it is not part of the Fortran standard and Intel Fortran does not provide it:

• Ubuntu / Intel (ifx): ifx compiles the call without error (it treats SYMLNK as an external procedure), but the linker then fails because there is no symlnk_ symbol in Intel's runtime library → undefined reference to 'symlnk_'.
• Windows / Intel (ifx): Intel Fortran on Windows recognises SYMLNK as a keyword with a different (Windows-specific) signature, so when it sees LOWERCASE_STRING(...) as a call argument it conflicts with its own internal declaration → compile-time errors #6406 Conflicting attributes and #6362 The data types of the argument(s) are invalid.
• Ubuntu / GFortran: GFortran documents and implements SYMLNK as a built-in extension, so it works fine there.

Fix applied (commit a6db0b7):

Replaced all 7 call symlnk(...) calls with a portable private helper create_symlink that uses ISO C binding to call the POSIX symlink() C function directly:

subroutine create_symlink(src, dst)
    use iso_c_binding, only: c_int, c_char, c_null_char
    character(len=*), intent(in) :: src, dst
#ifndef _WIN32
    interface
        function c_symlink(target, linkpath) bind(C, name="symlink") result(res)
            use iso_c_binding, only: c_int, c_char
            character(kind=c_char), intent(in) :: target(*), linkpath(*)
            integer(c_int) :: res
        end function c_symlink
    end interface
    integer(c_int) :: res
    res = c_symlink(trim(src)//c_null_char, trim(dst)//c_null_char)
    if (res /= 0) then
        call WarnErrReport('create_symlink: failed to create symlink from ' // &
                           trim(src) // ' to ' // trim(dst))
    end if
#endif
end subroutine create_symlink

This approach:

• Works with GFortran, Intel ifx/ifort, and any other Fortran 2003-compliant compiler — ISO C binding is standard
• No shell involved → no command-injection risk
• Checks the C return value and reports failures via WarnErrReport
• On Windows (_WIN32) the entire body is compiled out — the filesystem is case-insensitive so the symlink is unnecessary there anyway