Fix baremetal arm_compute_validation build errors

* Add -C flag to instruct preprocessor not to strip comments. This is to
prevent marker comments like '// fall through' that suppresses certain
warnings from being removed.

* Fix unused variable warnings.

* Add M_PI definition that's missing from certain toolchain standard
libraries.

Resolves COMPMID-4054

Change-Id: I1d641db668685d4b678f3d0efed84bfe9e630b4b
Signed-off-by: SiCongLi <sicong.li@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/4692
Reviewed-by: Georgios Pinitas <georgios.pinitas@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Tested-by: Arm Jenkins <bsgcomp@arm.com>
diff --git a/SConstruct b/SConstruct
index b543bc0..6b20ac2 100644
--- a/SConstruct
+++ b/SConstruct
@@ -166,6 +166,10 @@
 else:
     env.Append(CXXFLAGS = ['-Wlogical-op','-Wnoexcept','-Wstrict-null-sentinel'])
 
+if cpp_compiler == 'g++':
+    # Don't strip comments that could include markers
+    env.Append(CXXFLAGS = ['-C'])
+
 if env['cppthreads']:
     env.Append(CPPDEFINES = [('ARM_COMPUTE_CPP_SCHEDULER', 1)])