giflib library name should be giflib not libgif

[Henry Read]

giflib library name should be giflib not libgif.

OpenScenGraph 3.4.0's CMakeModule will not search for libgif.
" FIND_DEPENDENCY(GIFLIB gif_lib.h "ungif;libungif;giflib" ${OSG_3RDPARTY_BIN} "D" "")"
It only searches 'ungif;libungif;giflib'.
Project name is correct but target lib is not right.
It should be
"""
PROJECT(giflib C)

SET(TARGET_LIB giflib)

[Henry Read]

Please look at
https://github.com/openscenegraph/OpenSceneGraph/blob/master/CMakeModules/Find3rdPartyDependencies.cmake