fix weird interaction with cmake value checking

on windows, cmake is interpreting the empty string as a set value which
causes problems in eigen/blas/CMakeLists.txt. Instead, correlate the
value with a 'false'

eigen/CMakeLists.txt

@@ -36,7 +36,7 @@ if (ENABLE_EIGEN)
     -DBUILD_STATIC_LIBS=${BUILD_STATIC_LIBS}
     -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
     -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
-    -DCMAKE_Fortran_COMPILER=""
+    -DCMAKE_Fortran_COMPILER=OFF
     -DCMAKE_INSTALL_PREFIX=${CMAKE_NOBUNDLE_INSTALL_PREFIX}
     -DCMAKE_INSTALL_LIBDIR:PATH=${LIB_DIR}
     -DCMAKE_INSTALL_RPATH=${CMAKE_BUNDLE_INSTALL_PREFIX}/${LIB_DIR}