doc: update build instructions for OS X

Building Node.js on OS X requires XCode (because node-gyp requires
XCode). Add that information to BUILDING.md.

Additionally, this changes references to `Macintosh` in BUILDING.md to
refer to `OS X`. This is consistent with the way other references are to
operating system families (`Unix`, `Windows`) and not brand names or
hardware architectures.

PR-URL: #6309
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>

BUILDING.md

@@ -9,16 +9,25 @@ If you consistently can reproduce a test failure, search for it in the
 file a new issue.
 
 
-### Unix / Macintosh
+### Unix / OS X
 
 Prerequisites:
 
 * `gcc` and `g++` 4.8 or newer, or
 * `clang` and `clang++` 3.4 or newer
 * Python 2.6 or 2.7
 * GNU Make 3.81 or newer
+
+On OS X, you will also need:
+* [Xcode](https://developer.apple.com/xcode/download/)
+  * You also need to install the `Command Line Tools` via Xcode. You can find
+    this under the menu `Xcode -> Preferences -> Downloads`
+  * This step will install `gcc` and the related toolchain containing `make`
+
+On FreeBSD and OpenBSD, you may also need:
 * libexecinfo (FreeBSD and OpenBSD only)
 
+
 ```text
 $ ./configure
 $ make
@@ -133,7 +142,7 @@ This option will build with "small" (English only) support, but
 the full `Intl` (ECMA-402) APIs.  With `--download=all` it will
 download the ICU library as needed.
 
-##### Unix / Macintosh:
+##### Unix / OS X:
 
 ```text
 $ ./configure --with-intl=small-icu --download=all
@@ -156,7 +165,7 @@ data at runtime.
 With the `--download=all`, this may download ICU if you don't have an
 ICU in `deps/icu`.
 
-##### Unix / Macintosh:
+##### Unix / OS X:
 
 ```text
 $ ./configure --with-intl=full-icu --download=all
@@ -173,7 +182,7 @@ $ ./configure --with-intl=full-icu --download=all
 The `Intl` object will not be available. This is the default at
 present, so this option is not normally needed.
 
-##### Unix / Macintosh:
+##### Unix / OS X:
 
 ```text
 $ ./configure --with-intl=none
@@ -185,7 +194,7 @@ $ ./configure --with-intl=none
 > vcbuild intl-none
 ```
 
-#### Use existing installed ICU (Unix / Macintosh only):
+#### Use existing installed ICU (Unix / OS X only):
 
 ```text
 $ pkg-config --modversion icu-i18n && ./configure --with-intl=system-icu
@@ -201,7 +210,7 @@ You can find other ICU releases at
 Download the file named something like `icu4c-**##.#**-src.tgz` (or
 `.zip`).
 
-##### Unix / Macintosh
+##### Unix / OS X
 
 ```text
 # from an already-unpacked ICU: