Make scripts capable of 32-bit Windows builds

Now binaries can be (cross-)built using GCC for 32- and 64-bit Windows.

Additional improvements:
 - Within MSYS2, try to install the required dependencies automatically.
 - Within MSYS2, fix passing libdir paths to pkg-config.
 - Prune documentation from extracted package files,
   addressing the incredible slowness of Windows filesystem operations.
 - Fix the script name in README.adoc instructions.

tiffer.h

@@ -161,7 +161,7 @@ static bool
 tiffer_subifd(
 	const struct tiffer *self, uint32_t offset, struct tiffer *subreader)
 {
-	if (self->end - self->begin < offset)
+	if (self->begin > self->end || self->end - self->begin < offset)
 		return false;
 
 	*subreader = *self;