dependencies: boost: use system include flag for header includes

... based on the compiler object

This addresses issue #1569

Need to be careful about using -isystem with the standard include
dirs (thanks to the unit tests for catching this).  It may be
worthwhile trying to detect what the include dirs are.

Other dependencies (GTest) just avoid adding the include dir for those
system includes.  Do the same here.

mesonbuild/dependencies.py

@@ -675,13 +675,38 @@ class BoostDependency(Dependency):
 
     def get_compile_args(self):
         args = []
+        include_dir = ''
         if self.boost_root is not None:
             if mesonlib.is_windows():
-                args.append('-I' + self.boost_root)
+                include_dir = self.boost_root
             else:
-                args.append('-I' + os.path.join(self.boost_root, 'include'))
+                include_dir = os.path.join(self.boost_root, 'include')
         else:
-            args.append('-I' + self.incdir)
+            include_dir = self.incdir
+
+        # Use "-isystem" when including boost headers instead of "-I"
+        # to avoid compiler warnings/failures when "-Werror" is used
+
+        # Careful not to use "-isystem" on default include dirs as it
+        # breaks some of the headers for certain gcc versions
+
+        # For example, doing g++ -isystem /usr/include on a simple
+        # "int main()" source results in the error:
+        # "/usr/include/c++/6.3.1/cstdlib:75:25: fatal error: stdlib.h: No such file or directory"
+
+        # See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70129
+        # and http://stackoverflow.com/questions/37218953/isystem-on-a-system-include-directory-causes-errors
+        # for more details
+
+        # TODO: The correct solution would probably be to ask the
+        # compiler for it's default include paths (ie: "gcc -xc++ -E
+        # -v -") and avoid including those with -isystem
+
+        # For now, use -isystem for all includes except for some
+        # typical defaults (which don't need to be included at all
+        # since they are in the default include paths)
+        if include_dir != '/usr/include' and include_dir != '/usr/local/include':
+            args.append("".join(self.cpp_compiler.get_include_args(include_dir, True)))
         return args
 
     def get_requested(self, kwargs):