Revert an accidentally merged change [ci skip]

spec/ruby/optional/capi/spec_helper.rb

@@ -33,6 +33,12 @@ def compile_extension(name)
   ruby_header = "#{rubyhdrdir}/ruby.h"
   abi_header = "#{rubyhdrdir}/ruby/internal/abi.h"
 
+  if RbConfig::CONFIG["ENABLE_SHARED"] == "yes"
+    # below is defined since 2.1, except for mswin, and maybe other platforms
+    libdirname = RbConfig::CONFIG.fetch 'libdirname', 'libdir'
+    libruby = "#{RbConfig::CONFIG[libdirname]}/#{RbConfig::CONFIG['LIBRUBY']}"
+  end
+
   begin
     mtime = File.mtime(lib)
   rescue Errno::ENOENT
@@ -43,6 +49,7 @@ def compile_extension(name)
     when mtime <= File.mtime("#{spec_ext_dir}/#{ext}.c")
     when mtime <= File.mtime(ruby_header)
     when (mtime <= File.mtime(abi_header) rescue nil)
+    when libruby && mtime <= File.mtime(libruby)
     else
       return lib # up-to-date
     end