cc65: fix HEAD install; gcc.mak no longer exists.

The `-f make/gcc.mak` option is still required for the latest stable release, but `make/gcc.mak` was made unnecessary and removed in https://github.com/cc65/cc65/commit/9fece990bea6525bec5afd8b60d1c731a4d13839 in 2013, so providing that option for HEAD builds causes an error.

Unsuccessful HEAD install without this patch:

```
$ brew install --HEAD cc65
==> Cloning https://github.com/cc65/cc65.git
Updating /Library/Caches/Homebrew/cc65--git
==> make -f make/gcc.mak prefix=/usr/local/Cellar/cc65/HEAD libdir=/usr/local/Cellar/cc65/HEAD/share
prefix=/usr/local/Cellar/cc65/HEAD
libdir=/usr/local/Cellar/cc65/HEAD/share

make: make/gcc.mak: No such file or directory
make: *** No rule to make target `make/gcc.mak'.  Stop.
```

Successful installation from HEAD with this patch:

```
$ brew install --HEAD cc65                                                                                                                                                        
==> Cloning https://github.com/cc65/cc65.git
Updating /Library/Caches/Homebrew/cc65--git
==> make prefix=/usr/local/Cellar/cc65/HEAD libdir=/usr/local/Cellar/cc65/HEAD/share
==> make install prefix=/usr/local/Cellar/cc65/HEAD libdir=/usr/local/Cellar/cc65/HEAD/share
==> Caveats
Library files have been installed to:
  /usr/local/Cellar/cc65/HEAD/share/cc65
==> Summary
🍺  /usr/local/Cellar/cc65/HEAD: 354 files, 23M, built in 87 seconds
brew install --HEAD cc65  48.42s user 27.89s system 87% cpu 1:27.66 total

$ brew test cc65
Testing cc65
==> /usr/local/Cellar/cc65/2.13.3/bin/cl65 foo.c
```

Successful install from stable tarball with this patch:

```
$ brew install cc65
==> Downloading ftp://ftp.musoftware.de/pub/uz/cc65/cc65-sources-2.13.3.tar.bz2
Already downloaded: /Library/Caches/Homebrew/cc65-2.13.3.tar.bz2
==> make -f make/gcc.mak prefix=/usr/local/Cellar/cc65/2.13.3 libdir=/usr/local/Cellar/cc65/2.13.3/sh
==> make -f make/gcc.mak install prefix=/usr/local/Cellar/cc65/2.13.3 libdir=/usr/local/Cellar/cc65/2
==> Caveats
Library files have been installed to:
  /usr/local/Cellar/cc65/2.13.3/share/cc65
==> Summary
🍺  /usr/local/Cellar/cc65/2.13.3: 241 files, 14M, built in 71 seconds
brew install cc65  42.95s user 18.71s system 86% cpu 1:11.31 total

$ brew test cc65
Testing cc65
==> /usr/local/Cellar/cc65/2.13.3/bin/cl65 foo.c
```