Fix build on Intel's compiler suite.

Fix build on Intel's compiler suite.

Forcing 64-bit file operations on 64-bit systems exposes
an issue in stdlibc++, which is covered by gcc but not by
icc in non-gcc mode. This patch prevents forcing 64-bit
file operations on systems where long int is 64-bit or more.
It also properly protects a couple of gcc-specific #pragma.