meson: use pkg-config method to find dependencies

We do not need to ask cmake for the dependencies, so just use the
pkg-config mechanism.  Keep "auto" for SDL so that it tries using
sdl-config too.

The documentation is adjusted to use SDL2_image as the example,
rather than SDL which does not use the "pkg-config" method.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/meson.build b/meson.build
index e8a0f12..55c7d23 100644
--- a/meson.build
+++ b/meson.build
@@ -129,7 +129,7 @@
 pixman = not_found
 if have_system or have_tools
   pixman = dependency('pixman-1', required: have_system, version:'>=0.21.8',
-                      static: enable_static)
+                      method: 'pkg-config', static: enable_static)
 endif
 pam = not_found
 if 'CONFIG_AUTH_PAM' in config_host
@@ -168,7 +168,7 @@
   xkbcommon = not_found
 else
   xkbcommon = dependency('xkbcommon', required: get_option('xkbcommon'),
-                         static: enable_static)
+                         method: 'pkg-config', static: enable_static)
 endif
 slirp = not_found
 if config_host.has_key('CONFIG_SLIRP')
@@ -247,7 +247,7 @@
   sdl = declare_dependency(compile_args: '-Wno-undef',
                            dependencies: sdl)
   sdl_image = dependency('SDL2_image', required: get_option('sdl_image'),
-                         static: enable_static)
+                         method: 'pkg-config', static: enable_static)
 else
   if get_option('sdl_image').enabled()
     error('sdl-image required, but SDL was @0@',
@@ -332,7 +332,7 @@
 if get_option('vnc').enabled()
   vnc = declare_dependency() # dummy dependency
   png = dependency('libpng', required: get_option('vnc_png'),
-                   static: enable_static)
+                   method: 'pkg-config', static: enable_static)
   jpeg = cc.find_library('jpeg', has_headers: ['jpeglib.h'],
                          required: get_option('vnc_jpeg'),
                          static: enable_static)