configure: Support pkg-config for zlib

This is needed for builds with the mingw64-* packages from Cygwin,
but also works for Linux.

Move the zlib test also more to the end because users should
get information on the really important missing packages
(which also require zlib) first.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20180712192603.11599-1-sw@weilnetz.de>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

configure

@@ -2151,23 +2151,6 @@ EOF
   fi
 fi
 
-#########################################
-# zlib check
-
-if test "$zlib" != "no" ; then
-    cat > $TMPC << EOF
-#include <zlib.h>
-int main(void) { zlibVersion(); return 0; }
-EOF
-    if compile_prog "" "-lz" ; then
-        :
-    else
-        error_exit "zlib check failed" \
-            "Make sure to have the zlib libs and headers installed."
-    fi
-fi
-LIBS="$LIBS -lz"
-
 ##########################################
 # lzo check
 
@@ -3479,6 +3462,29 @@ if ! compile_prog "$glib_cflags -Werror" "$glib_libs" ; then
     fi
 fi
 
+#########################################
+# zlib check
+
+if test "$zlib" != "no" ; then
+    if $pkg_config --exists zlib; then
+        zlib_cflags=$($pkg_config --cflags zlib)
+        zlib_libs=$($pkg_config --libs zlib)
+        QEMU_CFLAGS="$zlib_cflags $QEMU_CFLAGS"
+        LIBS="$zlib_libs $LIBS"
+    else
+        cat > $TMPC << EOF
+#include <zlib.h>
+int main(void) { zlibVersion(); return 0; }
+EOF
+        if compile_prog "" "-lz" ; then
+            LIBS="$LIBS -lz"
+        else
+            error_exit "zlib check failed" \
+                "Make sure to have the zlib libs and headers installed."
+        fi
+    fi
+fi
+
 ##########################################
 # SHA command probe for modules
 if test "$modules" = yes; then