drop -Wno-long-double compiler flag

new GCC compilers do not accept "obsolete and totally removed
in gcc 4.2 and later" -Wno-long-double flag

actually, it used to be Apple-only GCC extension for gcc<=3.3

Change-Id: Ied3320cbd45915682b628c99bb0a168ea4753bb7
Reviewed-on: https://gerrit.libreoffice.org/11819
Reviewed-by: Douglas Mencken <dougmencken@gmail.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>