added nim.cfg with web assembly target

nim.cfg

@@ -0,0 +1,42 @@
+@if asmjs or wasm:
+  d:emscripten
+@end
+
+@if emscripten or asmjs or wasm:
+  o:"index.html"
+  @if not wasm:
+    d:asmjs
+  @end
+
+  cc = clang
+  clang.exe = "emcc"
+  clang.linkerexe = "emcc"
+  clang.options.linker = ""
+  cpu = "i386"
+  @if wasm:
+    passC = "-s WASM=1 -Iemscripten"
+    passL = "-s WASM=1 -Lemscripten -s ALLOW_MEMORY_GROWTH=1 "
+  @elif asmjs:
+    passC = "-s ASM_JS=1 -Iemscripten" #-s USE_PTHREADS=1
+    passL = "-s ASM_JS=1 -Lemscripten" #-s ALLOW_MEMORY_GROWTH=1"
+  @end
+
+  @if release:
+    passC %= "-O3"
+    passL %= "-O3"
+  @end
+
+  #Fix _setjmp/longjmp problem. https://irclogs.nim-lang.org/24-09-2017.html#12:19:50 
+  d:nimStdSetjmp              # https://irclogs.nim-lang.org/24-09-2017.html#20:13:18
+
+  #SDL
+  define:sdl_custom_log
+  dynlibOverride:"SDL2"
+  dynlibOverride:"SDL2_gfx"
+  dynlibOverride:"SDL2_image"
+  dynlibOverride:"SDL2_mixer"
+  dynlibOverride:"SDL2_ttf"
+  dynlibOverride:"SDL2_net"
+  #passL %= "-s USE_SDL=2 ../libSDL2_gfx.o ../libSDL2_image-png.o ../libSDL2_ttf.o --preload-file ../assets"
+  passL %= "-s USE_SDL=2 -s USE_SDL_GFX=2"
+@end