portability: don't use _Pragma with ICC

ICC defines __GNUC__ [1], but does not support GCC's _Pragma for
diagnostics.  As a matter of fact, I believe it does not support
_Pragma at all (only #pragma) [2].

Reported by Maxim Prohorenko.
https://savannah.gnu.org/support/index.php?108339

[1] https://software.intel.com/en-us/cpp-compiler-18.0-developer-guide-and-reference-gcc-compatibility-and-interoperability
[2] https://software.intel.com/en-us/cpp-compiler-18.0-developer-guide-and-reference-pragmas

* data/c.m4 (YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN): Exclude ICC from
the club.

THANKS

@@ -89,6 +89,7 @@ Martin Mokrejs            [email protected]
 Martin Nylin              [email protected]
 Matt Kraai                [email protected]
 Matt Rosing               [email protected]
+Maxim Prohorenko          [email protected]
 Michael Catanzaro         [email protected]
 Michael Felt              [email protected]
 Michael Hayes             [email protected]