Fix cmake script to compile as submodule

[maxvgi]

I have found that the PHP-CPP library cannot be compiled with cmake if it is used as git submodule.

The main problems:

CMakeLists.txt has syntax errors

It is not contain all necessary options and has syntax errors in static linking

The placement of header files

Library headers are placed in /include directory, the main header phpcpp.h includes it in the following way:

#include <phpcpp/visibility.h>

Even if I add directory /includes to header search paths, the compiler will wearch header in /includes/phpcpp directory.

That is not problem, if I compile the library with make and install it with make install, headers are placed in the system directories properly. But this becomes a problem if I do not want to install the library. Creating symlinks is the ugly way.

[yaroslavche]

Hello, @EmielBruijntjes !
First of all - thank you for your great library. It's very useful.
We faced one small (not critical) issue with it - we can't add PHP-CPP as submodule and must install library separately.
I'll be very appreciate if you had some time to review this PR and tell your opinion about that.