Escape double-quote in ConfigureChecks to fix CMake 2.8.12.1 warnings.

This warning occurs starting with CMake version efcf318f where the
CMake lexer has been taught how to understand line continuation for
quoted arguments.

Fixes #32
Merge branch '32-fix-cmake2.8.12.1-configure-warning'

* 32-fix-cmake2.8.12.1-configure-warning:
  Escape double-quote in ConfigureChecks to fix CMake 2.8.12.1 warnings.