Mix GNU GPL and MIT

[D504]

Some of source files are marked two licences at the same time.
For example:
GLSL.tab.c
GLSL.tab.h

How can it be? Which parts of files are GPL-licensed? Which parts of files are MIT-licensed?
May be I just don't understand licences, but GNU GPL is a viral license.

[Connicpu]

I Am Not A Lawyer, but this particular source appears to contain a special exception allowing them to license it under the MIT license, since the original work was used to create a parser, not a modified parser-generator.

[mseddon]

See http://www.gnu.org/software/bison/manual/html_node/Conditions.html