An attempt to only set both libz-related defines at the same time. We need

both the lib and the header present for both defines to be set. If only one
of the files is found, we issue a warning and set no define.

configure.ac

@@ -758,28 +758,60 @@ case "$OPT_ZLIB" in
     dnl check for the lib first without setting any new path, since many
     dnl people have it in the default path
 
-    AC_CHECK_LIB(z, inflateEnd, ,
+    AC_CHECK_LIB(z, inflateEnd,
+                   dnl libz found, set the variable
+                   [HAVE_LIBZ="1"],
+                   dnl if no lib found, try to add the given library
                    [if test -d "$OPT_ZLIB"; then
                       CPPFLAGS="$CPPFLAGS -I$OPT_ZLIB/include"
                       LDFLAGS="$LDFLAGS -L$OPT_ZLIB/lib"
                    fi])
 
-    AC_CHECK_HEADER(zlib.h,[
-      AC_CHECK_LIB(z, gzread,
-                   [HAVE_LIBZ="1"
-                   AC_SUBST(HAVE_LIBZ)
-                   AC_DEFINE(HAVE_ZLIB_H, 1, [if you have the zlib.h header file])
-                   AC_DEFINE(HAVE_LIBZ, 1, [If zlib is available])],
-                   [ CPPFLAGS=$_cppflags
-                   LDFLAGS=$_ldflags])],
-      [ CPPFLAGS=$_cppflags
-      LDFLAGS=$_ldflags]
-    )
+    AC_CHECK_HEADER(zlib.h,
+      [
+      dnl zlib.h was found
+      HAVE_ZLIB_H="1"
+      dnl if the lib wasn't found already, try again with the new paths
+      if test "$HAVE_LIBZ" != "1"; then
+        AC_CHECK_LIB(z, gzread,
+                     [
+                     dnl the lib was found!
+                     HAVE_LIBZ="1"
+                     ],
+                     [ CPPFLAGS=$_cppflags
+                     LDFLAGS=$_ldflags])
+      fi
+      ],
+      [
+        dnl zlib.h was not found, restore the flags
+        CPPFLAGS=$_cppflags
+        LDFLAGS=$_ldflags]
+      )
+
+    if test "$HAVE_LIBZ" = "1" && test "$HAVE_ZLIB_H" != "1"
+    then
+      AC_MSG_WARN([configure found only the libz lib, not the header file!])
+    elif test "$HAVE_LIBZ" != "1" && test "$HAVE_ZLIB_H" = "1"
+    then
+      AC_MSG_WARN([configure found only the libz header file, not the lib!])
+    elif test "$HAVE_LIBZ" = "1" && test "$HAVE_ZLIB_H" = "1"
+    then
+      dnl both header and lib were found!
+      AC_SUBST(HAVE_LIBZ)
+      AC_DEFINE(HAVE_ZLIB_H, 1, [if you have the zlib.h header file])
+      AC_DEFINE(HAVE_LIBZ, 1, [if zlib is available])
+
+      LIBS="$LIBS -lz"
+
+      dnl replace 'HAVE_LIBZ' in the automake makefile.ams
+      AMFIXLIB="1"
+      AC_MSG_NOTICE([found both libz and libz.h header])
+    fi
     ;;
 esac
 
 dnl set variable for use in automakefile(s)
-AM_CONDITIONAL(HAVE_LIBZ, test x"$HAVE_LIBZ" = x1)
+AM_CONDITIONAL(HAVE_LIBZ, test x"$AMFIXLIB" = x1)
 
 dnl Default is to try the thread-safe versions of a few functions
 OPT_THREAD=on