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

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

Fixes #2248.

Fixes -Ddefault_library=static not having any effect.

Cherry-Picked-From: bb9c07d8fe8b90c42ba81fb5bb6f8a9826252660

testsuite/reftests/meson.build

@@ -1,6 +1,6 @@
 testexecdir = join_paths(installed_test_bindir, 'reftests')
 
-libgtkreftestprivate = shared_library('gtkreftestprivate', [
+libgtkreftestprivate = library('gtkreftestprivate', [
   'reftest-compare.c',
   'reftest-module.c',
   'reftest-snapshot.c'],
@@ -483,4 +483,4 @@ if get_option('installed_tests')
   endforeach
 
   install_data(test_data, install_dir: testexecdir)
-endif
+endif