PHP-8.0 support

Bump version to 2.1.0

the Value::isCallable() did not work, it has been refactored based on…

… the "method_exists" implementation

Don't create a std::string with a nullptr

Bump version to 1.5.2

Revert "Revert "Revert "Fix issue with derived classes, but only fix …

…it for PHP 5.4 and up. Closes CopernicaMarketingSoftware#211"""

This reverts commit c512e02.

prepare version number for upcoming 1.5 release

Add soname and release build traget, default make target is debug

fixed a crash when dynamicly loaded extensions were unloaded, the sta…

…tic cache in PHP-CPP with all the extension names to extension pointers, and the extension numbers to extension pointers are now cleanup up when an extension is unloaded

added methods IniValue::stringValue(), IniValue::numericValue(), etc