libpq: Use macosx platform strlcpy() implementation

When compiling with libc++, it isn't necessary for libpq to provide
its own implementation of `strlcpy()`.  In fact, attempting to
build `strlcpy.h` fails because libc++ defines `strlcpy` to be a
macro.

Author: peter-b

libpq/libpq.gyp

@@ -127,6 +127,15 @@
 									},
 								}
 							],
+							[
+								'OS == "mac"',
+								{
+									'sources!':
+									[
+										'src/strlcpy.c',
+									],
+								},
+							],
 						],
 					},
 					{