CMake; windows: link to lowercase 'shlwapi' - consistent with headers (…

…#840)

The filenames are consistently inconsistent in windows world, might
have something to do with default file system being case-insensitive.
While the native MinGW buils were fixed in 5261307
that only addressed the headers, but not libraries.
The problem remains when one tries to do a MinGW cross-build from
case-sensitive filesystem.

src/CMakeLists.txt

@@ -44,7 +44,7 @@ endif()
 
 # We need extra libraries on Windows
 if(${CMAKE_SYSTEM_NAME} MATCHES "Windows")
-  target_link_libraries(benchmark Shlwapi)
+  target_link_libraries(benchmark shlwapi)
 endif()
 
 # We need extra libraries on Solaris