meson: honour meson warning_level option

Meson defines a warning_level option which has the following behaviour
with C code

 0:  no warning flags
 1: -Wall
 2: -Wall -Wextra
 3: -Wall -Wextra -Wpedantic

Currently we add our extra warning flags unconditionally if the compiler
supports them, regardless of the meson warning_level setting. This has
effectively nullified the warning_level setting in meson, and also
results in meson printing these messages:

  meson.build:498: WARNING: Consider using the built-in warning_level option instead of using "-Wall".
  meson.build:498: WARNING: Consider using the built-in warning_level option instead of using "-Wextra".

Semantically we can think of our huge list of flags as being an "extra"
set of warnings, and thus we ought to only add them when meson would
itself use -Wextra. aka warning_level == 2 or 3.

In practice libvirt code can't be built with -Wpedantic so we can ignore
meson warning_level 3, and only add our flags when warning_level==2.

In doing this change, we no longer have to check -Wall/-Wextra ourselves
as we can assume meson already set them.

-W is an alias of -Wextra so it is removed too.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

meson.build

@@ -229,12 +229,10 @@ cc_flags += [
   '-fexceptions',
   '-fipa-pure-const',
   '-fno-common',
-  '-W',
   '-Wabsolute-value',
   '-Waddress',
   '-Waddress-of-packed-member',
   '-Waggressive-loop-optimizations',
-  '-Wall',
   '-Walloc-size-larger-than=@0@'.format(alloc_max.stdout().strip()),
   '-Warray-bounds=2',
   '-Wattribute-alias=2',
@@ -268,7 +266,6 @@ cc_flags += [
   '-Wempty-body',
   '-Wendif-labels',
   '-Wexpansion-to-defined',
-  '-Wextra',
   '-Wformat-contains-nul',
   '-Wformat-extra-args',
   # -Wformat=2 implies -Wformat-nonliteral so we need to manually exclude it
@@ -398,6 +395,8 @@ cc_flags += [
   '-Wwrite-strings',
 ]
 
+supported_cc_flags = []
+if get_option('warning_level') == '2'
   supported_cc_flags = cc.get_supported_arguments(cc_flags)
 
   # on aarch64 error: -fstack-protector not supported for this target
@@ -463,6 +462,7 @@ if not cc.compiles(w_unused_function_code, args: w_unused_function_args)
     supported_cc_flags += ['-Wno-unused-function']
   endif
 
+endif
 add_project_arguments(supported_cc_flags, language: 'c')
 
 if cc.has_argument('-Wsuggest-attribute=format')