Improve checking for emscripten zlib

Change-Id: I52cff17d4d909135e40be53cd5c22a1565953762

configure.ac

@@ -7420,17 +7420,16 @@ dnl and has no pkg-config for it at least on some tinderboxes,
 dnl so leaving that out for now
 dnl libo_CHECK_SYSTEM_MODULE([zlib],[ZLIB],[zlib])
 AC_MSG_CHECKING([which zlib to use])
-if test "$_os" = "Emscripten"; then
-    # Emscripten provides its own zlib
-    AC_MSG_RESULT([Emscripten provided])
-    SYSTEM_ZLIB=TRUE
-elif test "$with_system_zlib" = "yes"; then
+if test "$with_system_zlib" = "yes"; then
     AC_MSG_RESULT([external])
     SYSTEM_ZLIB=TRUE
-    AC_CHECK_HEADER(zlib.h, [],
-        [AC_MSG_ERROR(zlib.h not found. install zlib)], [])
-    AC_CHECK_LIB(z, deflate, [ ZLIB=-lz ],
-        [AC_MSG_ERROR(zlib not found or functional)], [])
+    if test "$_os" != "Emscripten"; then
+        # Emscripten provides its own zlib, don't check for that
+        AC_CHECK_HEADER(zlib.h, [],
+            [AC_MSG_ERROR(zlib.h not found. install zlib)], [])
+        AC_CHECK_LIB(z, deflate, [ ZLIB=-lz ],
+            [AC_MSG_ERROR(zlib not found or functional)], [])
+    fi
 else
     AC_MSG_RESULT([internal])
     SYSTEM_ZLIB=