Add math library to executable link flags

Should fix bug #706882:

https://bugzilla.gnome.org/show_bug.cgi?id=706882
Fix return value of xsltRegisterExtModuleElement

xsltRegisterExtModuleElement returned success if xsltNewExtElement
failed. Detected by -Wunused-but-set-variable.
Remove unused variable in xsltGenerateIdFunction

doc isn't used as of commit "Fix generate-id() to avoid generating the
same ID".
Suppress unused variable warning.
Remove unused parameters
Fix time type in xsltSaveProfiling