Fix a number of problems with our serf configure detection.

build/ac-macros/serf.m4
  (): Update the description
  (SVN_LIB_SERF): Providing a prefix should trigger an error if serf isn't
    found.  Set a serf_prefix variable when a prefix is passed.  Shift running
    the tests outside the AC_ARG_WITH macro, it never belonged there anyway.
    Try with pkg-config before falling back on the prefix method and only
    use the prefix method if someone provided a prefix.
  (SVN_SERF_PREFIX_CONFIG): Don't use withval and instead use the new
    serf_prefix environment variable we set above.
  (SVN_SERF_PKG_CONFIG): Make our pkg-config handling capable of supporting
    prefixes, only accept the pkg-config detected library if it's the library
    in the prefix provided.  Support prefixes that point at the dir containing
    the pc file.  Only -l arguments belong in SVN_SERF_LIBS, fixes pkg-config
    builds failing with libraries not in the default search path for the
    linker.



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1572762 13f79535-47bb-0310-9956-ffa450edef68