Instead of printing the same message over and over again, if a target can't use
a specific sanitizer, the message will be printed only once for the first
affected target now.

If you like to preload a sanitized library in front of an application, it will
fail, because it is not the first in the library list anymore. This behaviour
is gcc specific - clang will static link the sanitizers instead. To get the
same for gcc, the new SANITIZE_LINK_STATIC flag will link the sanitizers
static in gcc environments.

Note: The preloaded executable must be sanitized, too! Otherwise ASan will
      print an error message!

If a compiler does not support sanitizing, it will not be checked a second time.

* Added a new module FindSanitizers with new function add_sanitizers to combine
  all sanitizers in one function instead of adding each one by one.
* Code of FindASan.cmake was outdourced into helper functions, so that the code
  may be used by other sanitizer modules, too.
* AddressSanitizer will be used with -O1 now to get a better performance.

* removed build type ASAN. Targets should define ASan usage by sanitize_address
  function
* compiler flags will be searched by compiler instead of per language
* FindASan warns, if a target can't be sanitized because of incompatible
  compilers
* added some helper functions