mesa: change 'SHADER_SUBST' facility to work with env variables

mesa: change 'SHADER_SUBST' facility to work with env variables

Patch modifies existing shader source and replace functionality to work
with environment variables rather than enable dumping on compile time.
Also instead of _mesa_str_checksum, _mesa_sha1_compute is used to avoid
collisions.

Functionality is controlled via two environment variables:

MESA_SHADER_DUMP_PATH - path where shader sources are dumped
MESA_SHADER_READ_PATH - path where replacement shaders are read

v2: cleanups, add strerror if fopen fails, put all functionality
    inside HAVE_SHA1 since sha1 is required

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Suggested-by: Eero Tamminen <eero.t.tamminen@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>