glsl_compiler: Re-enable building glsl_compiler

This allows application developers to use Mesa's compiler as a
standalone validator for their shaders.

This is mostly a revert of commit 569f0e4.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/SConscript

@@ -98,11 +98,11 @@ if env['platform'] == 'windows':
 
 env.Prepend(LIBS = [glsl])
 
-glsl2 = env.Program(
-    target = 'glsl2',
+glsl_compiler = env.Program(
+    target = 'glsl_compiler',
     source = compiler_objs,
 )
-env.Alias('glsl2', glsl2)
+env.Alias('glsl_compiler', glsl_compiler)
 
 glcpp = env.Program(
     target = 'glcpp/glcpp',