Fix BOOST_ROOT variable CMake warning (#21)

* Fix BOOST_ROOT variable CMake warning

* BOOST_ROOT variable to BOOST_PATH

Author: Oscar Torreno

.appveyor.yml

@@ -8,7 +8,7 @@ image:
     - Visual Studio 2017
 environment:
     GENERATOR: "Visual Studio 15 Win64"
-    BOOST_ROOT: C:\Libraries\boost_1_67_0
+    BOOST_PATH: C:\Libraries\boost_1_67_0
     AF_PATH: C:\Program Files\ArrayFire\v3
     KHIVA_PATH: C:\Program Files\Khiva\v0
     PATH: $(AF_PATH)\lib;$(KHIVA_PATH)\lib;C:\Python36-x64;C:\Python36-x64\Scripts;$(PATH)
@@ -42,7 +42,7 @@ before_build:
             cd C:\khiva-library;
             mkdir build;
             cd build;
-            cmake .. -DCMAKE_TOOLCHAIN_FILE=C:/tools/vcpkg/scripts/buildsystems/vcpkg.cmake -DKHIVA_USE_CONAN=OFF -DKHIVA_BUILD_DOCUMENTATION=OFF -DKHIVA_BUILD_BENCHMARKS=OFF -DKHIVA_BUILD_EXAMPLES=OFF -DKHIVA_BUILD_TESTS=OFF -DKHIVA_ONLY_CPU_BACKEND=ON -G"$env:GENERATOR";
+            cmake .. -DCMAKE_TOOLCHAIN_FILE=C:/tools/vcpkg/scripts/buildsystems/vcpkg.cmake -DBOOST_ROOT="$($env:BOOST_PATH)" -DKHIVA_USE_CONAN=OFF -DKHIVA_BUILD_DOCUMENTATION=OFF -DKHIVA_BUILD_BENCHMARKS=OFF -DKHIVA_BUILD_EXAMPLES=OFF -DKHIVA_BUILD_TESTS=OFF -DKHIVA_ONLY_CPU_BACKEND=ON -G"$env:GENERATOR";
             cmake --build . --config Release --target INSTALL -- /m;
           }
     - cd C:\khiva-java