More information is available in the gcc directly thread. These are the already building packages:

bigreqsproto-1.0.2  BigReqs extension headers from modular Xorg X11
kbproto-1.0.3       KB extension headers from X.org
xcmiscproto-1.1.2   XCMisc extension headers from X.org
xextproto-7.0.2     XExt extension headers from X.org
inputproto-1.4.2.1  Input extension headers from X.org
xf86bigfontproto-1.1.2 XF86BigFont extension headers from X.org
xproto-7.0.10       X protocol and ancillary headers from Xorg X11
xtrans-1.0.4        Network API translation layer to insulate X
libXau-1.0.3        Authorization Protocol for X from X.org
libXdmcp-1.0.2      X Display Manager Control Protocol library from X.org
ucpp-1.3            C preprocessor and lexer
libX11-1.1.3        Base X libraries from modular Xorg X11
fixesproto-4.0      Fixes extension headers from X.org
compositeproto-0.4  Composite extension headers from modular X.org
damageproto-1.1.0   Damage extension headers from modular X.org
evieext-1.0.2       EvIE extension headers
fontsproto-2.0.2    Fonts extension headers from X.org
randrproto-1.2.1    Randr extension headers from modular X.org
renderproto-0.9.2   Render extension headers from modular X.org
resourceproto-1.0.2 Resource extension headers from X.org
scrnsaverproto-1.1.0 ScrnSaver extension headers from X.org
libXext-1.0.3       X Extension library
libXi-1.1.3         X Input extension library
trapproto-3.4.3     Trap extension headers
videoproto-2.2.2    Video extension headers from modular X.org X11
xf86dgaproto-2.0.2  XF86DGA extension headers
glproto-1.4.8       GL extension headers