build: fix build errors on windows

* fix compiler issue where string was returned instead of wstring
* fix boost caching in CMakeList: for CMake 3.12.2 the "set" command requires a docstring if the CACHE flag is set

CMakeLists.txt

@@ -99,7 +99,7 @@ endif()
 # Boost ------------------------------------------------------------------------
 
 set(Boost_USE_MULTITHREAD ON)
-set(Boost_USE_STATIC_LIBS ON CACHE BOOL)
+set(Boost_USE_STATIC_LIBS ON CACHE BOOL "Set to ON to force the use of the static libraries.")
 set(Boost_USE_STATIC_RUNTIME OFF)
 
 find_package(Boost 1.68 COMPONENTS system program_options filesystem date_time REQUIRED)

src/lib_utility/utility/utilityString.cpp

@@ -602,7 +602,7 @@ std::wstring elide(const std::wstring& str, ElideMode mode, size_t size)
 		return str.substr(0, size - 3) + L"...";
 	}
 
-	return "";
+	return L"";
 }
 
 std::wstring convertWhiteSpacesToSingleSpaces(const std::wstring& str)