scons,gpu: Use a config header for the BUILD_GPU setting.

The BUILD_GPU setting was being set by adding a -D to the command line
at the top level SConstruct. Instead, add BUILD_GPU to export_vars so
that it ends up in a config/build_gpu.hh header. Also switch it from the
fairly dangerous #ifdef style to the safer #if.

Change-Id: Ic93d37c9d7671023a6978842dbb2750c3e92f8d8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40873
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -809,8 +809,8 @@ sticky_vars.AddVariables(
 
 # These variables get exported to #defines in config/*.hh (see src/SConscript).
 export_vars += ['USE_FENV', 'TARGET_ISA', 'TARGET_GPU_ISA',
-                'USE_POSIX_CLOCK', 'USE_KVM', 'USE_TUNTAP', 'PROTOCOL',
-                'HAVE_PROTOBUF', 'HAVE_VALGRIND',
+                'USE_POSIX_CLOCK', 'USE_KVM', 'USE_TUNTAP', 'BUILD_GPU',
+                'PROTOCOL', 'HAVE_PROTOBUF', 'HAVE_VALGRIND',
                 'HAVE_PERF_ATTR_EXCLUDE_HOST', 'USE_PNG',
                 'NUMBER_BITS_PER_SET', 'USE_HDF5']
 
@@ -1028,9 +1028,6 @@ Build variables for {dir}:
             warning("Can't connect EtherTap with a tap device.")
             env['USE_TUNTAP'] = False
 
-    if env['BUILD_GPU']:
-        env.Append(CPPDEFINES=['BUILD_GPU'])
-
     # Warn about missing optional functionality
     if env['USE_KVM']:
         if not main['HAVE_PERF_ATTR_EXCLUDE_HOST']: