Set macOS LIBRARY_PATH for Homebrew

Per the macOS ld(1) man page <https://www.unix.com/man-page/OSX/1/ld/>,

> The default library search path is /usr/lib then /usr/local/lib.

When `HOMEBREW_PREFIX=/usr/local`, this is where the `.dylib` files for
X11 are symlinked by Homebrew.

When `AC_PATH_XTRA` is called, it uses `/usr/bin/gcc` (Apple clang) and
since the libraries are found in `/usr/local/lib`, it does not need to
specify any `-L` paths.

However when `libtool` is used to call Homebrew `gfortran`, it sets
the flag `-syslibroot` which per the man page for ld(1):

> The -syslibroot option will prepend a prefix to all search paths.

and this means that it no longer looks under `/usr/local/lib` by
default. To fix this, add the default path back by setting the
`LIBRARY_PATH` environment variable.

.github/workflows/ci.yml

@@ -30,6 +30,7 @@ jobs:
           brew install automake \
             libpng \
             libxt
+          echo "LIBRARY_PATH=$(brew --prefix)/lib${LIBRARY_PATH:+:${LIBRARY_PATH}}" >> $GITHUB_ENV
       - name: Bootstrap
         run: |
           sh ./bootstrap