use pkg-config to locate zlib rather than hardcoding system -L/-I paths

for pthreads, just hardcode it enabled

for gnutls, don't hardcode -L/-I all the time to the host paths

--- a/lib/configure.ac
+++ b/lib/configure.ac
@@ -70,26 +70,23 @@ AC_ARG_WITH(zlib, AS_HELP_STRING([--without-zlib],
 AC_MSG_CHECKING([whether to include zlib compression support])
 if test x$ac_zlib != xno; then
  AC_MSG_RESULT(yes)
- AC_LIB_HAVE_LINKFLAGS(z,, [#include <zlib.h>], [compress (0, 0, 0, 0);])
- if test "$ac_cv_libz" != yes; then
-   AC_MSG_WARN(
-*** 
-*** ZLIB was not found. You will not be able to use ZLIB compression.)
- fi
 else
  AC_MSG_RESULT(no)
 fi
 
-PKG_CHECK_EXISTS(zlib, ZLIB_HAS_PKGCONFIG=y, ZLIB_HAS_PKGCONFIG=n)
-
 if test x$ac_zlib != xno; then
+  PKG_CHECK_MODULES(ZLIB, zlib, ZLIB_HAS_PKGCONFIG=y, ZLIB_HAS_PKGCONFIG=n)
   if test "$ZLIB_HAS_PKGCONFIG" = "y" ; then
+    LTLIBZ=$ZLIB_LIBS
+    AC_DEFINE([HAVE_LIBZ], [1], [zlib is enabled])
+    AC_SUBST(LTLIBZ)
     if test "x$GNUTLS_REQUIRES_PRIVATE" = "x"; then
       GNUTLS_REQUIRES_PRIVATE="Requires.private: zlib"
     else
       GNUTLS_REQUIRES_PRIVATE="$GNUTLS_REQUIRES_PRIVATE , zlib"
     fi
   else
+    AC_LIB_HAVE_LINKFLAGS(z,, [#include <zlib.h>], [compress (0, 0, 0, 0);])
     GNUTLS_ZLIB_LIBS_PRIVATE="$LTLIBZ"
   fi
 fi
@@ -124,10 +121,12 @@ fi
 lgl_INIT
 
 AC_CHECK_FUNCS(getrusage,,)
-AC_LIB_HAVE_LINKFLAGS(pthread,, [#include <pthread.h>], [pthread_mutex_lock (0);])
+LTLIBPTHREAD='-pthread'
+AC_DEFINE([HAVE_LIBPTHREAD], [1], [pthread enabled])
+AC_SUBST(LTLIBPTHREAD)
 
-LIBGNUTLS_LIBS="-L${libdir} -lgnutls $LIBS"
-LIBGNUTLS_CFLAGS="-I${includedir}"
+LIBGNUTLS_LIBS="-lgnutls $LIBS"
+LIBGNUTLS_CFLAGS=""
 AC_SUBST(LIBGNUTLS_LIBS)
 AC_SUBST(LIBGNUTLS_CFLAGS)
 
--- a/libextra/configure.ac
+++ b/libextra/configure.ac
@@ -41,8 +41,8 @@ LIBGNUTLS_EXTRA_HOOKS
 
 xgl_INIT
 
-LIBGNUTLS_EXTRA_LIBS="-L${libdir} -lgnutls-extra $LZO_LIBS $LIBGNUTLS_LIBS"
-LIBGNUTLS_EXTRA_CFLAGS="-I${includedir}"
+LIBGNUTLS_EXTRA_LIBS="-lgnutls-extra $LZO_LIBS $LIBGNUTLS_LIBS"
+LIBGNUTLS_EXTRA_CFLAGS=""
 AC_SUBST(LIBGNUTLS_EXTRA_LIBS)
 AC_SUBST(LIBGNUTLS_EXTRA_CFLAGS)