m4macros: replace obsolete macros AC_TRY_RUN and AC_TRY_LINK in gtk-2.0.m4

Running autoconf 2.71 with -Wall,error on a configure.ac that uses AM_PATH_GTK_2_0 gives:

configure.ac:268: warning: The macro `AC_TRY_RUN' is obsolete.
configure.ac:268: You should run autoupdate.
./lib/autoconf/general.m4:2997: AC_TRY_RUN is expanded from...
aclocal.m4:250: AM_PATH_GTK_2_0 is expanded from...
configure.ac:268: the top level
configure.ac:268: warning: The macro `AC_TRY_LINK' is obsolete.
configure.ac:268: You should run autoupdate.
./lib/autoconf/general.m4:2920: AC_TRY_LINK is expanded from...
aclocal.m4:250: AM_PATH_GTK_2_0 is expanded from...
configure.ac:268: the top level

Run autoupdate on gtk-2.0.m4 to change AC_TRY_RUN and AC_TRY_LINK into the suggested alternative, and adjust the formatting a little bit.

This was based on the same logic already done to glib-2.0.m4

glib@6f26637e