GN: Don't write ldflags and libs when unneeded.

tools/gn/variables.cc

Index: tools/gn/variables.cc
diff --git a/tools/gn/variables.cc b/tools/gn/variables.cc
index c88a182fb6359f1a4d10cb77116738662c1e1411..c0bf6cbc9a34b47737a9649fcde2a58d4b72a729 100644
--- a/tools/gn/variables.cc
+++ b/tools/gn/variables.cc
@@ -937,24 +937,35 @@ const char kLibs_Help[] =
     "\n"
     "  A list of library names or library paths.\n"
     "\n"
-    "Values containing '/' will be treated as references to files in the\n"
-    "build. They will be rebased to be relative to the build directory and\n"
-    "specified in the \"libs\" for linker tools. This facility should be used\n"
-    "for libraries that are checked in to the build. For libraries that are\n"
-    "generated by the build, use normal GN deps to link them.\n"
-    "\n"
-    "Values not containing '/' will be treated as system library names. These\n"
-    "will be passed unmodified to the linker and prefixed with the\n"
-    "\"lib_prefix\" attribute of the linker tool. Generally you would set the\n"
-    "\"lib_dirs\" so the given library is found. Your BUILD.gn file should\n"
-    "not specify the switch prefix (like \"-l\"): this will be encoded in\n"
-    "the \"lib_prefix\" of the tool.\n"
-    "\n"
-    "  Libraries ending in \".framework\" will be special-cased: the switch\n"
-    "  \"-framework\" will be prepended instead of the lib_prefix, and the\n"
-    "  \".framework\" suffix will be trimmed. This is to support the way Mac\n"
-    "  links framework dependencies.\n"
+    "  These libraries will be linked into the final binary (executable or\n"
+    "  shared library) containing the current target.\n"
     COMMON_LIB_INHERITANCE_HELP
+    "\n"
+    "Types of libs\n"
+    "\n"
+    "  There are several different things that can be expressed in libs:\n"
+    "\n"
+    "  File paths\n"
+    "      Values containing '/' will be treated as references to files in\n"
+    "      the checkout. They will be rebased to be relative to the build\n"
+    "      directory and specified in the \"libs\" for linker tools. This\n"
+    "      facility should be used for libraries that are checked in to the\n"
+    "      version control. For libraries that are generated by the build,\n"
+    "      use normal GN deps to link them.\n"
+    "\n"
+    "  System libraries\n"
+    "      Values not containing '/' will be treated as system library names.\n"
+    "      These will be passed unmodified to the linker and prefixed with\n"
+    "      the \"lib_prefix\" attribute of the linker tool. Generally you\n"
+    "      would set the \"lib_dirs\" so the given library is found. Your\n"
+    "      BUILD.gn file should not specify the switch (like \"-l\"): this\n"
+    "      will be encoded in the \"lib_prefix\" of the tool.\n"
+    "\n"
+    "  Apple frameworks\n"
+    "      System libraries ending in \".framework\" will be special-cased:\n"
+    "      the switch \"-framework\" will be prepended instead of the\n"
+    "      lib_prefix, and the \".framework\" suffix will be trimmed. This is\n"
+    "      to support the way Mac links framework dependencies.\n"
     COMMON_ORDERING_HELP
     LIBS_AND_LIB_DIRS_ORDERING_HELP
     "\n"
@@ -962,6 +973,7 @@ const char kLibs_Help[] =
     "\n"
     "  On Windows:\n"
     "    libs = [ \"ctl3d.lib\" ]\n"
+    "\n"
     "  On Linux:\n"
     "    libs = [ \"ld\" ]\n";