cmake: update FindGTK2 to v3.0.1-1824-g0b12815

Current FindGTK2.cmake as included with Wireshark cannot do a 32-bit
build even after setting CMAKE_LIBRARY_PATH=/usr/lib32. After an update
to upstream FindGTK2 (v3.0.1-1824-g0b12815), things build again.

Compared to upstream FindGTK2, FindWSWinLibs and related hints have been
added (note that "${GTK2_HINTS}/lib" is not a typo, it is needed as some
headers are includes in locations like lib/gtk-2.0/include/). The
include paths have become relative again as there are no files like
FindPackageHandleStandardArgs in the Wireshark source tree. Diff between
upstream FindGTK2 and the version in this patch:

    diff --git a/cmake/modules/FindGTK2.cmake b/cmake/modules/FindGTK2.cmake
    index 72bb8eb..9b4f989 100644
    --- a/cmake/modules/FindGTK2.cmake
    +++ b/cmake/modules/FindGTK2.cmake
    @@ -167,2 +167,5 @@

    +include( FindWSWinLibs )
    +FindWSWinLibs( "gtk2" "GTK2_HINTS" )
    +
     #=============================================================
    @@ -176,4 +179,4 @@

    -include(${CMAKE_CURRENT_LIST_DIR}/SelectLibraryConfigurations.cmake)
    -include(${CMAKE_CURRENT_LIST_DIR}/CMakeParseArguments.cmake)
    +include(SelectLibraryConfigurations)
    +include(CMakeParseArguments)

    @@ -289,2 +292,5 @@ function(_GTK2_FIND_INCLUDE_DIR _var _hdr)
                 ${_suffixes}
    +        HINTS
    +            "${GTK2_HINTS}/include"
    +            "${GTK2_HINTS}/lib"
         )
    @@ -395,2 +401,4 @@ function(_GTK2_FIND_LIBRARY _var _lib _expand_vc _append_version)
                 [HKEY_LOCAL_MACHINE\\SOFTWARE\\gtkmm\\2.4;Path]/lib
    +        HINTS
    +            "${GTK2_HINTS}/lib"
             )
    @@ -409,2 +417,4 @@ function(_GTK2_FIND_LIBRARY _var _lib _expand_vc _append_version)
                 [HKEY_LOCAL_MACHINE\\SOFTWARE\\gtkmm\\2.4;Path]/lib
    +        HINTS
    +            "${GTK2_HINTS}/lib"
             )
    @@ -829,3 +839,3 @@ set(_GTK2_did_we_find_everything true)  # This gets set to GTK2_FOUND

    -include(${CMAKE_CURRENT_LIST_DIR}/FindPackageHandleStandardArgs.cmake)
    +include(FindPackageHandleStandardArgs)

Change-Id: I02103409a79ea30d1bf7cc0dfb43a9e41f8a4db3
Reviewed-on: https://code.wireshark.org/review/4293
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>
(cherry picked from commit fa1d0be4bbd21f06a3e2aa7fdab2182118fa9c52)
Reviewed-on: https://code.wireshark.org/review/4400
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Michael Mann <mmann78@netscape.net>