msvc: split multi-token -I/-L values from pkg-config

[quaesitor-scientiam]

Summary

Fix MSVC handling of pkg-config output where -I/-L values contain multiple tokens
(e.g. -Ifoo -Ibar -Lbaz) by splitting and applying them individually.

What was wrong

pkg-config sometimes returns multiple -I/-L tokens inside a single flag value.
The MSVC builder previously wrapped the entire value into one /I"..." or /LIBPATH:"..."
argument, so embedded -I/-L tokens were passed verbatim inside the string.
MSVC ignored/misparsed those, causing missing include paths (e.g. glibconfig.h).

What changed

The -I and -L branches now split multi-token values and apply each part separately.
This prevents embedded GNU-style tokens from being smuggled inside a single MSVC argument.

Repro

Build a GUI example on Windows with MSVC and vcpkg glib/pango installed; previously
glibconfig.h was missing even though include paths were present.

Tests

Not run (local build of arrows.v succeeded).

Changed files

• vlib/v/builder/msvc_windows.v

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