CMake: use CMAKE_REQUIRED_INCLUDES to find APR macros

When using CMake with Visual Studio on Windows, invoking the
CHECK_SYMBOL_EXISTS macro with the full paths to the include files seems
to always result in failure.

Instead, use the documented CMAKE_REQUIRED_INCLUDES variable to set the
include directory, and pass only the headers' base names to
CHECK_SYMBOL_EXISTS.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1752331 13f79535-47bb-0310-9956-ffa450edef68