installdeps: support gentoo

Add a check for gentoo and the appropriate emerge commands to get the
library and toolchain dependencies.

installdeps

@@ -159,6 +159,8 @@ linux_installdeps() {
         archlinux_installdeps
     elif [ -f /etc/solus-release ]; then
         solus_installdeps
+    elif [ -f /etc/gentoo-release ]; then
+        gentoo_installdeps
     elif [ -f /etc/os-release ]; then
         case "$(. /etc/os-release; echo "$ID_LIKE")" in
             *suse*)
@@ -728,6 +730,16 @@ solus_installdeps() {
     build_instructions
 }
 
+gentoo_installdeps() {
+    installing
+
+    check sudo emerge-webrsync
+    check sudo emerge -vuDUa gcc make cmake binutils libsdl2 libsfml wxGTK zlib dev-util/pkgconfig nasm
+    check sudo eselect wxwidgets set 1
+
+    build_instructions
+}
+
 windows_installdeps() {
     msys2=1