diff --git a/meson.build b/meson.build
index d6401fb8e3..f9279c837d 100644
--- a/meson.build
+++ b/meson.build
@@ -1946,6 +1946,38 @@ if cc.get_id() == 'msvc'
 endif
 
 
+# Compute flags that are built into Meson.  We need these to
+# substitute into Makefile.global and for pg_config.  We only compute
+# the flags for Unix-style compilers, since that's the only style that
+# would use Makefile.global or pg_config.
+
+# We don't use get_option('warning_level') here, because the other
+# warning levels are not useful with PostgreSQL source code.
+common_builtin_flags = ['-Wall']
+
+if get_option('debug')
+  common_builtin_flags += ['-g']
+endif
+
+optimization = get_option('optimization')
+if optimization == '0'
+  common_builtin_flags += ['-O0']
+elif optimization == '1'
+  common_builtin_flags += ['-O1']
+elif optimization == '2'
+  common_builtin_flags += ['-O2']
+elif optimization == '3'
+  common_builtin_flags += ['-O3']
+elif optimization == 's'
+  common_builtin_flags += ['-Os']
+endif
+
+cflags_builtin = cc.get_supported_arguments(common_builtin_flags)
+if llvm.found()
+  cxxflags_builtin = cpp.get_supported_arguments(common_builtin_flags)
+endif
+
+
 
 ###############################################################
 # Atomics
diff --git a/src/include/meson.build b/src/include/meson.build
index a28f115d86..58b7a9c1e7 100644
--- a/src/include/meson.build
+++ b/src/include/meson.build
@@ -44,9 +44,9 @@ config_paths_data.set_quoted('MANDIR', dir_prefix / dir_man)
 
 var_cc = ' '.join(cc.cmd_array())
 var_cpp = ' '.join(cc.cmd_array() + ['-E'])
-var_cflags = ' '.join(cflags + cflags_warn + get_option('c_args'))
+var_cflags = ' '.join(cflags + cflags_builtin + cflags_warn + get_option('c_args'))
 if llvm.found()
-  var_cxxflags = ' '.join(cxxflags + cxxflags_warn + get_option('cpp_args'))
+  var_cxxflags = ' '.join(cxxflags + cxxflags_builtin + cxxflags_warn + get_option('cpp_args'))
 else
   var_cxxflags = ''
 endif