Change CC to cc on OpenBSD

[larryhynes]

rdrview compiles on OpenBSD but only when the Makefile is edited to make CC := cc.

~ $ uname
OpenBSD

~ $ which egcc
which: egcc: Command not found.

2 ~ $ which gcc
which: gcc: Command not found.

2 ~ $ which cc
/usr/bin/cc

[Veraellyunjie]

1. ### BSDs section of README.md mentions you need to install stuff including gcc.

2. 2 ~ $ which cc /usr/bin/cc only works if you have comp file set installed, which you probably have, but that's not always the case.

3. Is rdrview as good with clang as it is with gcc?
   Hopefully it is, but knowing for sure would be desirable before switching.

4. If rdrview is as good with clang as it is with gcc,
   then ### BSDs section of README.md needs reworking.
   OpenBSD and FreeBSD ship clang.
   NetBSD and DragonFlyBSD ship gcc.
   As of now, gcc is the compiler that works across *BSD.
   Just removing it or changing it to clang or cc would confuse NetBSD and DragonFlyBSD users.
   So README.md needs edits.
   Furthermore, Makefile needs edits setting clang or cc for FreeBSD as well