[sane_install #67] Also install include paths to given headers per target
[sane_install #67] Fixes the installed include path

It should point to the generic include path, so that when you install
headers to foo/header_subdir/bar.h the include path is foo, and not
foo/header_subdir
[sane_install #67] Fixes the installed include path

It should point to the generic include path, so that when you install
headers to e.g. /home/bob/include/foo/header_subdir/bar.h the include
path is /home/bob/include, and not /home/bob/include/foo
[sanitizers] Reworked all the sanitizer code to be the same, except for their compilation flags of course
[sanitizers] Add memory_sanitizer to sanitizers.cmake, gives warnings instead of errors when two are enabled

I opt for warnings over FATAL_ERROR since perhaps one day they will be
both supported, and we check for compatability anyway.