fixes entry for musl libc build, rather than glibc

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

Issues reported:
  - build procedures utilize col for Guidebook-creation, and col
    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'.

Related to GitHub #1393

doc/fixes3-7-0.txt

@@ -2440,6 +2440,8 @@ Unix: re-do command line parsing
 Unix: add ../include/nhlua.h to the alloc.o dependencies in Makefile.utl to
 	match Makefile.src
 Unix: implement SELF_RECOVER compile-time option, on by default on linux
+Unix: allow build to succeed with musl (instead of glibc), by specifying
+	musl=1 on the make command line
 user_sounds: move the message hook from inside individual window display ports
 	to the core where it allows MSGTYP_NOSHOW msgtyp's to still trigger
 	sounds to correct a reported github issue; also fixes a past reported