Update UNIX configure script for gcc7 and above.

Changes.txt

@@ -109,7 +109,8 @@
     both libraries are now compiled into the app whenever one is selected.
     This fixes issues with a newer ZLIB not working with an older PNG, etc.
 
-  * Updated UNIX configure script to work with the clang 5.x compiler.
+  * Updated UNIX configure script to work with the clang 5+ and gcc 7+
+    compiler versions.
 
 -Have fun!
 

configure

@@ -417,7 +417,7 @@ elif test "$have_gcc" = yes; then
 	fi
 
 	case $cxx_version in
-		4.[7-9]|4.[7-9].[0-9]|4.[7-9].[0-9][-.]*|[5-6].[0-9]|[5-6].[0-9].[0-9]|[5-6].[0-9].[0-9][-.]*)
+		4.[7-9]|4.[7-9].[0-9]|4.[7-9].[0-9][-.]*|[5-9]|[5-9].[0-9]|[5-9].[0-9].[0-9]|[5-9].[0-9].[0-9][-.]*)
 			_cxx_major=`echo $cxx_version | cut -d '.' -f 1`
 			_cxx_minor=`echo $cxx_version | cut -d '.' -f 2`
 			cxx_version="$cxx_version, ok"