back x11 configure tests

[Eric Wasylishen]

Hi,
I was getting a missing symbol error when I tried to drag and drop. It turns 
out that back wasn't linked to libXext; the configure test AC_CHECK_LIB(Xext, 
main) was failing (silently). This is on Ubuntu 10.10.

I changed it to: (and did the same for Xt and Xmu)

  AC_CHECK_LIB(Xext, main)
  if test "$HAVE_LIBXext" = no; then
    PKG_CHECK_MODULES([XEXT], [xext])
  fi

so it uses pkg-config if the AC_CHECK_LIB can't find the library. This fixed 
the problem for me, but I'm not sure if it's the right thing to do.

One thing is that according to the autoconf docs, PKG_CHECK_MODULES will cause 
a hard failure if it can't find the library. Are Xext, Xt, and Xmu hard 
dependencies of x11 back?

Cheers
Eric