glsl: Fix cross-version linking between VS and GS.

glsl: Fix cross-version linking between VS and GS.

Previously, when attempting to link a vertex shader and a geometry
shader that use different GLSL versions, we would sometimes generate a
link error due to the implicit declaration of gl_PerVertex being
different between the two GLSL versions.

This patch fixes that problem by only requiring interface block
definitions to match when they are explicitly declared.

Fixes piglit test "shaders/version-mixing vs-gs".

Cc: "10.0" <mesa-stable@lists.freedesktop.org>
v2: In the interface_block_definition constructor, move the assignment
to explicitly_declared after the existing if block.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>