disk_cache: build option for disabled-by-default

On some systems it is problematic to have the shader cache enabled
by default. This adds a build option to support the disk cache but
keep it disabled unless the environment variable
MESA_GLSL_CACHE_DISABLE=false.

For example, on Chrome OS, Chrome already has it's own shader
disk cache implementation so it disables the mesa feature. Tests
do not want the shader disk cache enabled because it can cause
inconsistent performance results and the default 1GB for the
disk cache could lead to problems that require more effort to
work around. The Mesa shader disk cache is useful for VMs though,
where it is easy to configure the feature with environment
variables. With the current version of Mesa, Chrome OS would need
to have a system-wide environment variable to disable the disk
cache everywhere except where needed. More elegant to just build
Mesa with the cache feature disabled by default.

Reviewed-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <mesa/mesa!6967>