Add compiler and configure info to mutt -v output (closes #3537)

Add compiler and configure info to mutt -v output (closes #3537)

Makefile(.am) updated to produce conststrings.c, which contains
C strings representing:

* the compiler's own version information;
* the CFLAGS value from the Make environment
* the ./configure options

main.c is updated to print them when running 'mutt -v'.

txt2c.sh is added to produce conststrings.c. txt2c.sh uses a compiled
txt2c binary if possible, for complete fidelity to the source strings
in whatever encoding they may use.  If txt2c is not available (could
not be compiled, or was not compiled natively) it falls back on a shell
function to approximate the output using sed and tr.