dl_open and friends are only used by ICU plugins that we don't use.
Nacl-related targets (with newlib) fail because dlfcn.h is missing.

Setting U_ENABLE_DYLOAD to 0, we don't have to worry about dlfcn.h any more (it's not included)

BUG=132145
TEST=After running build/gyp_chromium, build 'base_i18n_nacl' target. It should succeed without complaining about dlfcn.h.
R=sergeyu@chromium.org

Review URL: https://codereview.chromium.org/263763002

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/icu52@267357 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Update icu.gyp and related files to list C/C++ source files and header files.

Besides, HAVE_DLOPEN is set to 0 explicitly in icu.gyp. It's 0 by default
in ICU 4.6.1 but is 1 by default in ICU 52.1, but we don't need it. (it's only
necessary for ICU plugins that Chromium does not use).

I also applied https://codereview.chromium.org/218153002/ (Nico's CL to
simplify icu.gyp).

BUG=132145
TEST=None
R=mark@chromium.org, thakis@chromium.org

Review URL: https://codereview.chromium.org/212623010

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/icu52@260941 4ff67af0-8c30-449e-8e8b-ad334ec8d88c