Support LinuxMint in the build script

LinuxMint is based on Ubuntu, so can be handled with the
same package list as Debian and Ubuntu.

https://bugzilla.gnome.org/show_bug.cgi?id=596447

tools/build/gnome-shell-build-setup.sh

@@ -74,7 +74,7 @@ dpkg_is_installed() {
     return 1
 }
 
-if test x$system = xUbuntu -o x$system = xDebian ; then
+if test x$system = xUbuntu -o x$system = xDebian -o x$system = xLinuxMint ; then
   reqd=""
   for pkg in \
     build-essential curl \