Don't dynamically link to libuv on macOS

[nealrichardson]

Fixes #374

The linker on macOS always prefers to dynamically link, so if you have both shared and static versions of a library, the shared will be picked. To force static linking yet still use system libuv if found, this PR copies libuv.a to a different directory and renames it to libuv_static.a, and that's what gets passed in the PKG_LIBS.

Before, you see it linking to the dylib:

% R CMD INSTALL .
* installing to library ‘~/Library/R/arm64/4.3/library’
* installing *source* package ‘httpuv’ ...
** using staged installation
** Using libuv found by pkg-config in /opt/homebrew/Cellar/libuv/1.44.2
** PKG_CFLAGS=-I/opt/homebrew/Cellar/libuv/1.44.2/include
** PKG_LIBS=-L/opt/homebrew/Cellar/libuv/1.44.2/lib -luv
** libs...
* DONE (httpuv)

% otool -L ~/Library/R/arm64/4.3/library/httpuv/libs/httpuv.so | grep libuv
	/opt/homebrew/opt/libuv/lib/libuv.1.dylib (compatibility version 2.0.0, current version 2.0.0)

After, there is no link to the dylib:

% R CMD INSTALL .        
* installing to library ‘~/Library/R/arm64/4.3/library’
* installing *source* package ‘httpuv’ ...
** using staged installation
** Using static libuv found by pkg-config in /opt/homebrew/Cellar/libuv/1.44.2
** PKG_CFLAGS=-I/opt/homebrew/Cellar/libuv/1.44.2/include
** PKG_LIBS=-L/Users/npr/code/httpuv/src/libuv/.libs -luv_static
...

% otool -L ~/Library/R/arm64/4.3/library/httpuv/libs/httpuv.so | grep libuv

%

To confirm the fallback behavior, I deleted libuv.a, so pkg-config will still find libuv, but the static library isn't there. It correctly switches to the bundled build:

cp: /opt/homebrew/Cellar/libuv/1.44.2/lib/libuv.a: No such file or directory
** pkg-config did not find static libuv
** Using bundled copy of libuv
** PKG_CFLAGS=-Ilibuv/include
** PKG_LIBS=./libuv/.libs/libuv.a