Fix bug #552: disable EIGEN_GLIBC_MALLOC_ALREADY_ALIGNED when compiling with -fsanitize=address, and allow users to manually tell whether EIGEN_MALLOC_ALREADY_ALIGNED.

doc/PreprocessorDirectives.dox

@@ -30,7 +30,7 @@ are doing.
    initialized to zero, as are new entries in matrices and arrays after resizing. Not defined by default.
  - \b EIGEN_INITIALIZE_MATRICES_BY_NAN - if defined, all entries of newly constructed matrices and arrays are
    initialized to NaN, as are new entries in matrices and arrays after resizing. This option is especially
-   useful for debugging purpose, though a memory tool like <a href="http://valgrind.org/">valgring</a> is
+   useful for debugging purpose, though a memory tool like <a href="http://valgrind.org/">valgrind</a> is
    preferable. Not defined by default.
  - \b EIGEN_NO_AUTOMATIC_RESIZING - if defined, the matrices (or arrays) on both sides of an assignment 
    <tt>a = b</tt> have to be of the same size; otherwise, %Eigen automatically resizes \c a so that it is of
@@ -55,6 +55,9 @@ run time. However, these assertions do cost time and can thus be turned off.
 
 \section TopicPreprocessorDirectivesPerformance Alignment, vectorization and performance tweaking
 
+ - \b EIGEN_MALLOC_ALREADY_ALIGNED - Can be set to 0 or 1 to tell whether default system malloc already
+   returns aligned buffers. In not defined, then this information is automatically deduced from the compiler
+   and system preprocessor tokens.
  - \b EIGEN_DONT_ALIGN - disables alignment completely. %Eigen will not try to align its objects and does not
    expect that any objects passed to it are aligned. This will turn off vectorization. Not defined by default.
  - \b EIGEN_DONT_ALIGN_STATICALLY - disables alignment of arrays on the stack. Not defined by default, unless