Allow building cvtt with system squish
Allow building cvtt with system squish

It looks like the SCsub for cvtt was copied from squish and it left
the `if env['build_squish']:` line intact. This means that using
`scons builtin_squish=no modules/cvtt` would fail and overal builds
would also fail because they'd fail to find `ConvectionKernels.h`

BastiaanOlij/godot,pkowal1982/godot,BastiaanOlij/godot,guilhermefelipecgs/godot,godotengine/godot,Paulloz/godot,DmitriySalnikov/godot,Zylann/godot,ex/godot,vnen/godot,Valentactive/godot,vnen/godot,godotengine/godot,sanikoyes/godot,DmitriySalnikov/godot,Faless/godot,Paulloz/godot,Zylann/godot,pkowal1982/godot,vkbsb/godot,godotengine/godot,Valentactive/godot,josempans/godot,Shockblast/godot,Zylann/godot,Shockblast/godot,guilhermefelipecgs/godot,pkowal1982/godot,ex/godot,godotengine/godot,godotengine/godot,Faless/godot,Paulloz/godot,Valentactive/godot,sanikoyes/godot,MarianoGnu/godot,Shockblast/godot,honix/godot,MarianoGnu/godot,Valentactive/godot,Shockblast/godot,DmitriySalnikov/godot,honix/godot,BastiaanOlij/godot,firefly2442/godot,josempans/godot,vkbsb/godot,Faless/godot,firefly2442/godot,josempans/godot,ex/godot,firefly2442/godot,Valentactive/godot,firefly2442/godot,guilhermefelipecgs/godot,MarianoGnu/godot,firefly2442/godot,ex/godot,Shockblast/godot,honix/godot,guilhermefelipecgs/godot,ZuBsPaCe/godot,akien-mga/godot,ZuBsPaCe/godot,Faless/godot,BastiaanOlij/godot,pkowal1982/godot,sanikoyes/godot,ZuBsPaCe/godot,BastiaanOlij/godot,vnen/godot,Shockblast/godot,sanikoyes/godot,vnen/godot,Paulloz/godot,ZuBsPaCe/godot,Zylann/godot,vkbsb/godot,firefly2442/godot,sanikoyes/godot,akien-mga/godot,pkowal1982/godot,Valentactive/godot,Zylann/godot,vnen/godot,pkowal1982/godot,MarianoGnu/godot,Faless/godot,MarianoGnu/godot,akien-mga/godot,guilhermefelipecgs/godot,MarianoGnu/godot,BastiaanOlij/godot,vkbsb/godot,Shockblast/godot,Valentactive/godot,vkbsb/godot,ZuBsPaCe/godot,ZuBsPaCe/godot,ex/godot,vnen/godot,DmitriySalnikov/godot,akien-mga/godot,vnen/godot,Zylann/godot,akien-mga/godot,josempans/godot,honix/godot,josempans/godot,BastiaanOlij/godot,Faless/godot,akien-mga/godot,Paulloz/godot,honix/godot,ex/godot,akien-mga/godot,Faless/godot,pkowal1982/godot,ZuBsPaCe/godot,Paulloz/godot,Zylann/godot,josempans/godot,godotengine/godot,DmitriySalnikov/godot,Faless/godot,vkbsb/godot,MarianoGnu/godot,guilhermefelipecgs/godot,Paulloz/godot,Shockblast/godot,guilhermefelipecgs/godot,honix/godot,firefly2442/godot,Zylann/godot,Valentactive/godot,godotengine/godot,josempans/godot,akien-mga/godot,sanikoyes/godot,josempans/godot,DmitriySalnikov/godot,MarianoGnu/godot,DmitriySalnikov/godot,BastiaanOlij/godot,ZuBsPaCe/godot,ex/godot,vkbsb/godot,vkbsb/godot,godotengine/godot,firefly2442/godot,pkowal1982/godot,ex/godot,guilhermefelipecgs/godot,vnen/godot,sanikoyes/godot,sanikoyes/godot