gdal.c: add .so/.dll for GDAL 3.0

lib/raster/gdal.c

@@ -105,6 +105,7 @@ static void load_library(void)
 {
     static const char *const candidates[] = {
 # ifdef __unix__
+	"libgdal.so.26", /* GDAL 3.0 */
 	"libgdal.so.20",
 	"libgdal.so.1",
 	"libgdal.1.1.so",
@@ -115,6 +116,7 @@ static void load_library(void)
 	"libgdal1.7.0.so",
 # endif
 # ifdef _WIN32
+	"gdal300.dll",
 	"gdal204.dll",        
 	"gdal203.dll",        
 	"gdal202.dll",