musl libc build, rather than glibc

We've had reports of a couple of issues building against musl libc.

Issues reported:
  - build procedures utilize cat for Guidebook-creation, and cat
    is deprecated in distros that use musl libc.

  - some of the CRASHREPORT code is using library functions that
    are not available in the musl libc environment. The reported
    functions were backtrace() and backtrace_symbols(), which use
    header file /usr/include/execinfo.h.

So we'll try to accommodate this. Since we don't have a means of
autodetecting the musl libc situation during the build (as of yet), the
builder will have to specify 'make musl=1' on the make command line.

Specifying 'musl=1' on the make command line will:
1. ensure that NOCRASHREPORT gets defined in the C preprocessor.
2. set COLCMD to be '../util/stripbs' instead of 'col -bx'.

Closes #1393

sys/unix/Makefile.top

@@ -416,6 +416,12 @@ fetch-docs:
 	  shift; \
 	done
 
+makedefs:
+	( cd util ; $(MAKE) makedefs )
+
+stripbs:
+	( cd util ; $(MAKE) stripbs )
+
 # 'make update' can be used to install a revised version after making
 # customizations or such.  Unlike 'make install', it doesn't delete everything
 # from the target directory to have a clean start.