Misleading panic in gdal-sys build script

[ttencate]

This looks dodgy:

gdal/gdal-sys/build.rs

Lines 131 to 136 in 0747fa6

	if cfg!(target_env = "msvc") {
	panic!("windows-gnu requires gdal_i.lib to be present in either $GDAL_LIB_DIR or $GDAL_HOME\\lib.");
	}
	// otherwise, look for a gdalxxx.dll in $GDAL_HOME/bin
	// works in windows-gnu

I suppose the panic text should say windows-msvc instead of windows-gnu, but for all I know it might be the other way around, which would mean the condition and subsequent comment are wrong.

[lnicola]

Fixed in #332.