zlib.sh: flatten if tree, add missed commit message [ci skip]

Follow-up to a34bcc4
in which this change slipped in by accident.

It adapts to some of the build changes expected in the next zlib releases.
Untested.

zlib.sh

@@ -23,14 +23,12 @@ _VER="$1"
     options+=' -DZLIB_ENABLE_TESTS=OFF'
     options+=' -DZLIBNG_ENABLE_TESTS=OFF'
     options+=' -DWITH_GTEST=OFF'
+  elif [ "${_VER}" = '1.3.1' ]; then
+    options+=' -DZLIB_BUILD_EXAMPLES=OFF'
   else
-    if [ "${_VER}" = '1.3.1' ]; then
-      options+=' -DZLIB_BUILD_EXAMPLES=OFF'
-    else
-      options+=' -DZLIB_BUILD_TESTING=OFF'
-      options+=' -DZLIB_BUILD_SHARED=OFF'
-      options+=' -DZLIB_BUILD_MINIZIP=OFF'
-    fi
+    options+=' -DZLIB_BUILD_TESTING=OFF'
+    options+=' -DZLIB_BUILD_SHARED=OFF'
+    options+=' -DZLIB_BUILD_MINIZIP=OFF'
   fi
 
   # `BUILD_SHARED_LIBS=OFF` broken as of zlib v1.3.