calc: C-style arbitrary precision system Files

The following are the changes in this release:

    Requiring calc shell scripts to use -s -f at the end of the
    initial #! line.

    Fixed /tmp/mersenne example in calc(1) man page.

    Added make variable ${ARCH_CFLAGS}.  The ${ARCH_CFLAGS} is
    added after ${CCMISC} and before ${EXTRA_CFLAGS} when building
    the ${CFLAGS} for compiling C code.  are ${CC} when compiling
    C files.  The default value is:

	ARCH_CFLAGS= -march=native

    which directs C compiler to compile for the native machine.
    To disable use of '-march=native', set ARCH_CFLAGS to the empty
    string as in:

	make all ARCH_CFLAGS=

    To make calc RPMs more portable, they are compiled with an
    empty ARCH_CFLAGS.

    Fixed issues relating to compiling on macOS.  Fixed issues
    where <unistd.h> is needed.

    Fixed typos in help/intro and README.md. <<GitHub guilhermgonzaga>>

    Copied missing description lines from help/intro to README.md
    "What is calc?" section.  <<GitHub guilhermgonzaga>>

    GCC 7 added a warning on fall throughs in case statements.  It's
    enabled by -Wextra and treated as an error due to -Wall so it
    breaks compilation.  See -Wimplicit-fallthrough in the GCC
    manual.  The default value is 3, which means a comment matching
    some specific regexes is enough to disable the warning.
    Fixed spaces vs tabs and use FALLTHRU as it's used elsewhere.
    Fixed one FALLTHRU comment that was inconsistent with others.
    <<thanks jcul>>

    Fixed minor typo on help/power. <<GitHub thegithubr>>

    By default, the calc history file is located in ~/.calc_history.
    Now, if the environment variable $CALCHISTFILE is defined
    and is non-empty, then calc history file will be defined
    by the $CALCHISTFILE environment variable.

    Calc as defined 0^0 as 1.  However in the past, 0 raised to
    an expression that evaluted to zero returned 1.  The result
    was that 0^0 was different than 0^(6-6) or even 0^(0).
    Now, calc will return 1 for 0^(0) and 0^zero when zero == 0.