scons: Pull domain specific build setup out of SConstruct.

Use SConsopts files local to individual domains to pull
non-foundational build code out of SConstruct. This greatly simplifies
SConstruct, and also makes it easier to find build configuration having
to do with particular pieces of gem5.

This change also converts some python level variables, all_protocols,
protocol_dirs, and slicc_includes, into the environment where the timing
of their initialization is more flexible.

Change-Id: Ie61ceb75ae9e5557cc400603c972a9582e99c1ea
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40872
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>

src/base/SConsopts

@@ -0,0 +1,76 @@
+# Copyright 2020 Google, Inc.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met: redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer;
+# redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution;
+# neither the name of the copyright holders nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+Import('*')
+
+from gem5_scons import warning
+
+import gem5_scons
+
+conf = gem5_scons.Configure(main)
+
+# Check for <fenv.h> (C99 FP environment control)
+have_fenv = conf.CheckHeader('fenv.h', '<>')
+
+# Check for <png.h> (libpng library needed if wanting to dump
+# frame buffer image in png format)
+have_png = conf.CheckHeader('png.h', '<>')
+
+have_posix_clock = \
+    conf.CheckLibWithHeader([None, 'rt'], 'time.h', 'C',
+                            'clock_nanosleep(0,0,NULL,NULL);')
+if not have_posix_clock:
+    warning("Can't find library for POSIX clocks.")
+
+# Valgrind gets much less confused if you tell it when you're using
+# alternative stacks.
+main['HAVE_VALGRIND'] = conf.CheckCHeader('valgrind/valgrind.h')
+
+main = conf.Finish()
+
+
+if have_fenv:
+    sticky_vars.Add(BoolVariable('USE_FENV', 'Use <fenv.h> IEEE mode control',
+                                 True))
+else:
+    warning("Header file <fenv.h> not found.\n"
+            "This host has no IEEE FP rounding mode control.")
+    main['USE_FENV'] = False
+
+
+if have_png:
+    sticky_vars.Add(BoolVariable('USE_PNG',  'Enable support for PNG images',
+                                 True))
+else:
+    warning("Header file <png.h> not found.\n"
+            "This host has no libpng library.\n"
+            "Disabling support for PNG framebuffers.")
+    main['USE_PNG'] = False
+
+sticky_vars.Add(BoolVariable('USE_POSIX_CLOCK', 'Use POSIX Clocks',
+                             have_posix_clock))
+
+
+export_vars.extend(['USE_FENV', 'USE_PNG', 'USE_POSIX_CLOCK', 'HAVE_VALGRIND'])