[boost-test] Fix for VCPKG_BUILD_TYPE

[UnaNancyOwen]

If VCPKG_BUILD_TYPE is set, The error occurs because an empty directory is created.
This pull request will be fixed this error.

Starting package 82/91: boost-test:x64-windows
Building package boost-test[core]:x64-windows... 
-- CURRENT_INSTALLED_DIR=C:/Tools/vcpkg/installed/x64-windows
-- DOWNLOADS=C:/Tools/vcpkg/downloads
-- CURRENT_PACKAGES_DIR=C:/Tools/vcpkg/packages/boost-test_x64-windows
-- CURRENT_BUILDTREES_DIR=C:/Tools/vcpkg/buildtrees/boost-test
-- CURRENT_PORT_DIR=C:/Tools/vcpkg/ports/boost-test/.
-- Downloading https://github.com/boostorg/test/archive/boost-1.66.0.tar.gz...
-- Downloading https://github.com/boostorg/test/archive/boost-1.66.0.tar.gz... OK
-- Testing integrity of downloaded file...
-- Testing integrity of downloaded file... OK
-- Extracting source C:/Tools/vcpkg/downloads/boostorg-test-boost-1.66.0.tar.gz
-- Extracting done
-- Building x64-windows-rel
-- Building x64-windows-rel done
-- Packaging x64-windows-rel
-- Packaging x64-windows-rel done
-- Packaging headers
-- Packaging headers done
-- Using cached C:/Tools/vcpkg/downloads/boost_LICENSE_1_0.txt
-- Testing integrity of cached file...
-- Testing integrity of cached file... OK
-- Installing: C:/Tools/vcpkg/packages/boost-test_x64-windows/share/boost-test/copyright
-- Performing post-build validation
There should be no empty directories in C:/Tools/vcpkg/packages/boost-test_x64-windows
The following empty directories were found: 

    C:/Tools/vcpkg/packages/boost-test_x64-windows/debug/lib/manual-link

If a directory should be populated but is not, this might indicate an error in the portfile.
If the directories are not needed and their creation cannot be disabled, use something like this in the portfile to remove them:

    file(REMOVE_RECURSE ${CURRENT_PACKAGES_DIR}/a/dir ${CURRENT_PACKAGES_DIR}/some/other/dir)


Found 1 error(s). Please correct the portfile:
    C:\Tools\vcpkg\ports\boost-test\portfile.cmake
-- Performing post-build validation done
Error: Building package boost-test:x64-windows failed with: POST_BUILD_CHECKS_FAILED
Please ensure you're using the latest portfiles with `.\vcpkg update`, then
submit an issue at https://github.com/Microsoft/vcpkg/issues including:
  Package: boost-test:x64-windows
  Vcpkg version: 0.0.103-2018-01-27-340ff984cba132f71e4fabbc78c247ad4e4de292
Additionally, attach any relevant sections from the log files above.
Command exited with code 1

[alexkaratarakis]

Thanks!