Corrected BCrypt API detection check in CMakeLists.txt.

The check depends on Boost.Predef and Boost.WinAPI. Unfortunately,
we cannot use their CMake targets in check_cxx_source_compiles because
they may not yet be defined at the point of inclusion of
filesystem/CMakeLists.txt by the superproject. Thus we have to manually
specify include path to the monolithic include tree. And bcrypt library.

We intentionally do not specify include paths for Boost.Predef and
Boost.WinAPI because that would require to also specify paths of their
dependencies, and this is not an acceptable maintenance burden.

CMakeLists.txt

@@ -16,7 +16,12 @@ check_cxx_source_compiles("#include <${CMAKE_CURRENT_SOURCE_DIR}/config/has_stat
 check_cxx_source_compiles("#include <${CMAKE_CURRENT_SOURCE_DIR}/config/has_stat_st_mtimensec.cpp>" BOOST_FILESYSTEM_HAS_STAT_ST_MTIMENSEC)
 check_cxx_source_compiles("#include <${CMAKE_CURRENT_SOURCE_DIR}/config/has_stat_st_mtimespec.cpp>" BOOST_FILESYSTEM_HAS_STAT_ST_MTIMESPEC)
 if(WIN32)
-    check_cxx_source_compiles("#include <${CMAKE_CURRENT_SOURCE_DIR}/config/has_bcrypt.cpp" BOOST_FILESYSTEM_HAS_BCRYPT)
+    # Note: We can't use the Boost::library targets here as they may not yet be included by the superproject when this CMakeLists.txt is included.
+    set(CMAKE_REQUIRED_INCLUDES "${CMAKE_CURRENT_SOURCE_DIR}/../..")
+    set(CMAKE_REQUIRED_LIBRARIES bcrypt)
+    check_cxx_source_compiles("#include <${CMAKE_CURRENT_SOURCE_DIR}/config/has_bcrypt.cpp>" BOOST_FILESYSTEM_HAS_BCRYPT)
+    unset(CMAKE_REQUIRED_LIBRARIES)
+    unset(CMAKE_REQUIRED_INCLUDES)
 endif()
 
 add_library(boost_filesystem