build: Don't hardcode shared_library() in meson.

Instead, use the standard library().
This is a meson best practice.

Fixes -Ddefault_library=static not having any effect.

gtk/meson.build

@@ -1007,7 +1007,7 @@ if cc.has_header('langinfo.h')
 endif
 
 # Library
-libgtk = shared_library('gtk-4',
+libgtk = library('gtk-4',
                  soversion: gtk_soversion,
                  sources: [typefuncs, gtk_sources, gtkmarshal_h, gtkprivatetypebuiltins_h],
                  c_args: gtk_cargs + common_cflags,

testsuite/reftests/meson.build

@@ -1,9 +1,9 @@
 # FIXME: $(no_undefined) ?
-libgtkreftestprivate = shared_library('gtkreftestprivate',
+libgtkreftestprivate = library('gtkreftestprivate',
   'reftest-compare.c', 'reftest-module.c', 'reftest-snapshot.c',
   dependencies : libgtk_dep)
 
-libreftest = shared_library('reftest',
+libreftest = library('reftest',
   [ 'expand-expander.c',
     'frame-inhibitor.c',
     'letter-spacing.c',