Update Emscripten-build-instructions.rst

Author: mcbarton

docs/Emscripten-build-instructions.rst

@@ -151,7 +151,9 @@ or executing
                         -DCLANG_BUILD_TOOLS=OFF                         `
                         -G Ninja `
                         ..\llvm
-   $env:EMCC_CFLAGS="-Oz" emmake ninja libclang clangInterpreter clangStaticAnalyzerCore lldWasm
+   $env:EMCC_CFLAGS="-Oz" 
+   emmake ninja libclang clangInterpreter clangStaticAnalyzerCore lldWasm
+   $env:EMCC_CFLAGS=""
 
 on Windows. Once this finishes building we need to take note of where we built our llvm build.
 This can be done by executing the following on Linux and osx