Added warning when SDL2_DIR env var is not set on Windows.
Added warning when SDL2_DIR env var is not set on Windows.

jdryg/bgfx,Synxis/bgfx,mcanthony/bgfx,kondrak/bgfx,bkaradzic/bgfx,LWJGL-CI/bgfx,fluffyfreak/bgfx,jpcy/bgfx,0-wiz-0/bgfx,v3n/bgfx,septag/bgfx,ocornut/bgfx,marco-we/bgfx,fluffyfreak/bgfx,andr3wmac/bgfx,fluffyfreak/bgfx,emoon/bgfx,attilaz/bgfx,Extrawurst/bgfx,andr3wmac/bgfx,mmicko/bgfx,MikePopoloski/bgfx,bkaradzic/bgfx,sergeScherbakov/bgfx,marco-we/bgfx,aonorin/bgfx,Extrawurst/bgfx,Extrawurst/bgfx,sergeScherbakov/bgfx,cyndis/bgfx,BlueCrystalLabs/bgfx,elmindreda/bgfx,fluffyfreak/bgfx,ktotheoz/bgfx,mendsley/bgfx,attilaz/bgfx,aonorin/bgfx,darkimage/bgfx,ocornut/bgfx,jpcy/bgfx,v3n/bgfx,kondrak/bgfx,ktotheoz/bgfx,marco-we/bgfx,janstk/bgfx,elmindreda/bgfx,bkaradzic/bgfx,attilaz/bgfx,LSBOSS/bgfx,mmicko/bgfx,mmicko/bgfx,kondrak/bgfx,janstk/bgfx,darkimage/bgfx,sergeScherbakov/bgfx,janstk/bgfx,elmindreda/bgfx,aonorin/bgfx,cyndis/bgfx,Vertexwahn/bgfx,ming4883/bgfx,LWJGL-CI/bgfx,andr3wmac/bgfx,cuavas/bgfx,ktotheoz/bgfx,ocornut/bgfx,ming4883/bgfx,LSBOSS/bgfx,jpcy/bgfx,cyndis/bgfx,jdryg/bgfx,MikePopoloski/bgfx,jdryg/bgfx,jdryg/bgfx,cuavas/bgfx,emoon/bgfx,mcanthony/bgfx,mendsley/bgfx,LSBOSS/bgfx,LWJGL-CI/bgfx,Synxis/bgfx,0-wiz-0/bgfx,septag/bgfx,Vertexwahn/bgfx,BlueCrystalLabs/bgfx,Vertexwahn/bgfx,mendsley/bgfx,jpcy/bgfx,mcanthony/bgfx,BlueCrystalLabs/bgfx,v3n/bgfx,emoon/bgfx,cuavas/bgfx,septag/bgfx,LWJGL-CI/bgfx,0-wiz-0/bgfx,MikePopoloski/bgfx,ming4883/bgfx,bkaradzic/bgfx,darkimage/bgfx,Synxis/bgfx