Change use of AC_CHECK_LIB to AC_SEARCH_LIBS.

Change use of AC_CHECK_LIB to AC_SEARCH_LIBS.

Patch from Zack Weinberg.  His message:

  This one eliminates all use of AC_CHECK_LIB in the configure script.
  AC_CHECK_LIB has a serious flaw: if the library you mention *exists*
  but is not *necessary* for the function you want, it adds it to
  $(LIBS) anyway.  This was fine in the days of static libraries,
  because the linker would ignore an .a library that didn't contain
  anything you needed. However, ELF shared libraries are different
  (let's not get into why): the linker will by default record a
  DT_NEEDED entry for every shared object mentioned on the link
  command line. Thus, every use of AC_CHECK_LIB is a potential
  unnecessary DT_NEEDED, making extra work for the dynamic loader. The
  cure is simply to use AC_SEARCH_LIBS instead; it first tries to find
  the function you ask for in libc, and only if that doesn't work does
  it try to use the extra library you mention.

  For the same reasons, pkg-config .pc files should distinguish
  between the libraries to use for shared linkage (Libs:) and the
  additional libraries needed for static linkage (Libs.private:). I
  have also made that correction in this patch. I also took the
  opportunity to clean up the substitution variables a little and make
  absolutely sure that the core library does not get linked against
  zlib.

svn:r1338