README: update Linux-to-Windows cross-compilation instructions.

README.md

@@ -99,10 +99,10 @@ by passing the `-DENABLE_GUI=OFF` flag to the cmake invocation.
 
 ### Building for Windows
 
-You will need the usual build tools, CMake and a Windows cross-compiler.
+You will need the usual build tools, CMake, a Windows cross-compiler, and flatc.
 On a Debian derivative (e.g. Ubuntu) these can be installed with:
 
-    apt-get install git build-essential cmake mingw-w64
+    apt-get install git build-essential cmake mingw-w64 libflatbuffers-dev
 
 Before building, check out the project and the necessary submodules:
 
@@ -115,7 +115,8 @@ After that, build 32-bit SolveSpace as following:
     mkdir build
     cd build
     cmake .. -DCMAKE_TOOLCHAIN_FILE=../cmake/Toolchain-mingw32.cmake \
-             -DCMAKE_BUILD_TYPE=Release
+             -DCMAKE_BUILD_TYPE=Release \
+             -DFLATC=$(which flatc)
     make
 
 Or, build 64-bit SolveSpace as following:
@@ -123,7 +124,8 @@ Or, build 64-bit SolveSpace as following:
     mkdir build
     cd build
     cmake .. -DCMAKE_TOOLCHAIN_FILE=../cmake/Toolchain-mingw64.cmake \
-             -DCMAKE_BUILD_TYPE=Release
+             -DCMAKE_BUILD_TYPE=Release \
+             -DFLATC=$(which flatc)
     make
 
 The graphical interface is built as `build/bin/solvespace.exe`, and the command-line interface