stbi: add a link_to_libm.c.v helper, to ensure `import stbi` works always.

vlib/stbi/link_to_libm.c.v

@@ -0,0 +1,21 @@
+// Copyright (c) 2019-2021 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+module stbi
+
+// NB: stbi uses math.h because of `ldexp` and `pow`, for which we *do* have
+// pure V implementations, but our `math` module still depends on libm
+// because of 'powf', 'cosf', 'sinf', 'sqrtf' and 'tanf'.
+
+// TODO: remove this file, when we have pure V implementations for the above
+// functions too, and so `math` is no longer dependent on `libm` at all.
+
+#include <math.h>
+
+$if windows {
+	$if tinyc {
+		#flag @VEXEROOT/thirdparty/tcc/lib/openlibm.o
+	}
+} $else {
+	#flag -lm
+}