doc: Only CMAKE_INSTALL_PREFIX is needed.

Specifying also CMAKE_FIND_ROOT_PATH has no effect.

doc/building.dox

@@ -89,9 +89,9 @@ subdir (e.g. `C:/Sys/bin`) to PATH so all the DLLs are found when running the
 executables.  If you are using MinGW, the `C:/MinGW` directory is in most cases
 already prepared for exactly this.
 
-When running CMake, set `CMAKE_FIND_ROOT_PATH` and `CMAKE_INSTALL_PREFIX`
-parameters to that directory so CMake knows where to look for installed
-libraries and where to install new ones.
+When running CMake, set `CMAKE_INSTALL_PREFIX` parameter to that directory so
+CMake knows where to look for installed libraries and where to install new
+ones.
 
 @subsubsection building-windows-msvc Using Visual Studio
 
@@ -104,7 +104,7 @@ Studio to load:
 
     mkdir build
     cd build
-    cmake -DCMAKE_FIND_ROOT_PATH="C:/Sys" -DCMAKE_INSTALL_PREFIX="C:/Sys" ..
+    cmake -DCMAKE_INSTALL_PREFIX="C:/Sys" ..
     cmake --build .
     cmake --build --target install .