Set enable variables using cmake's logic

In cmake there are several different ways to specify true to an enable variable (i.e. ENABLE_PEXSI), such as on, ON, or yes. Right now, when trying to enable the optional solvers, the ELSI build system was just copying the value into the Fortran, and ELSI was checking the value of the string. But this didn't work if someone didn't write ON verbatim. I've modified the build system to pass an integer (0 or 1) instead. Now you can pass to cmake exotic lines like -DENABLE_EIGENEXA=on -DENABLE_SIPS=yes -DENABLE_PEXSI="Yes" -DENABLE_MAGMA="y" -DENABLE_BSEPACK="ON"