[eigen] check for old gcc (inadvertently?) catching intel compiler and disabling SSE

[Mark Borgerding <mark@xxxxxxxxxxxxxx>]

in Eigen/Core, there is a check that disables SSE for gcc compilers < 
4.2. Unfortunately it gets triggered under the *intel* compiler.

  #if (defined __SSE2__) && ( (!defined __GNUC__) || 
EIGEN_GNUC_AT_LEAST(4,2) )
    #define EIGEN_SSE2_ON_NON_MSVC_BUT_NOT_OLD_GCC
  #endif

The intel compiler masquerades by defining __GNUC__ and related version 
variables to mimic the gcc version installed on the system.
The effect in Eigen is that the intel compiler may not get a chance to 
use SSE even if it can[1].

e.g.

    [user@centos58-64 ~/]$ gcc --version
    gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-54)

    [user@centos58-64 ~/]$  icpc -dM -E - < /dev/null  |  grep GNUC
    #define __GNUC__ 4
    #define __GNUC_MINOR__ 1
    #define __GNUC_PATCHLEVEL__ 2

Intel compilers at least as recent as 12.1.3 20120212 exhibit this GNUC 
mimicry.


[1] What is the problem with gcc 4.1 and SIMD?  The version check in 
Eigen seems originate at changeset  246:566b4821dd1b , but I could not 
easily tell what problem it was fixing.
I have some systems still running RedHat 5.x, which has gcc 4.1 as the 
default compiler.  I'd like to support them with Eigen+SIMD on 
Intel(first choice), and/or platform-default gcc.


-- Mark