meson: Restore implicit warning/debug/optimize flags for extensions

Meson uses warning/debug/optimize flags such as "-Wall", "-g", and
"-O2" automatically based on "--warnlevel" and "--buildtype" options.
And we use "--warning_level=1" and "--buildtype=debugoptimized" by
default.

But we need these flags for Makefile.global (for extensions) and
pg_config, so we need to compute them manually based on the
higher-level options.

Without this change, extensions building using pgxs wouldn't get -Wall
or optimization options.

Author: Sutou Kouhei <kou@clear-code.com>
Discussion: https://www.postgresql.org/message-id/flat/20240122.141139.931086145628347157.kou%40clear-code.com

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

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