Some sanitizers are not compatible with other sanitizers. Added some checks to
avoid these combinations at configuration time.

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

According to the MemorySanitizer documentation, it is only compatible with 64bit
Linux at the moment.

According to the ThreadSanitizer documentation, it is only compatible with 64bit
Linux at the moment.

* 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

Thus C can be mixed with Fortran and the objects can be build with different
compilers, not both may support AddressSanitizer.

Instead of fix checking for C and CXX compilers, all enabled languages will be
tested now.

Older CMake versions are not capable of continue in foreach functions.