[meson] Normalize options names

As https://github.com/harfbuzz/harfbuzz/pull/2412#discussion_r426275027
diff --git a/meson.build b/meson.build
index fc4c16f..fa27929 100644
--- a/meson.build
+++ b/meson.build
@@ -189,11 +189,11 @@
   conf.set('HAVE_ICU', 1)
 endif
 
-if get_option('icu-builtin')
+if get_option('icu_builtin')
   conf.set('HAVE_ICU_BUILTIN', 1)
 endif
 
-if get_option('experimental-api')
+if get_option('experimental_api')
   conf.set('HB_EXPERIMENTAL_API', 1)
 endif