config: Use /bin/sh as default shell and don't check for its existence

/bin/bash is not available on every system, nor always the default.
/bin/sh is standardised, but not always the preferred interactive shell,
hence it's configurable.

There's no need to check for the existence of this shell during config,
since it's an option that only affects run-time - and you may compile
ncdu on a totally different system than you would run it on.

configure.ac

@@ -58,14 +58,10 @@ fi
 # Configure default shell for spawning shell when $SHELL is not set
 AC_ARG_WITH([shell],
   [AS_HELP_STRING([--with-shell],
-                  [used interpreter as default shell (default is /bin/bash)])],
+                  [used interpreter as default shell (default is /bin/sh)])],
                   [DEFAULT_SHELL=$withval],
-                  [DEFAULT_SHELL=/bin/bash])
+                  [DEFAULT_SHELL=/bin/sh])
 AC_MSG_NOTICE([Using $DEFAULT_SHELL as the default shell if \$SHELL is not set])
-AC_CHECK_PROG(TEST_SHELL, $DEFAULT_SHELL, yes, no, [/])
-if test "x$TEST_SHELL" != "xyes"; then
-  AC_MSG_ERROR([Default shell interpreter $DEFAULT_SHELL not found])
-fi
 AC_DEFINE_UNQUOTED(DEFAULT_SHELL, "$DEFAULT_SHELL", [Used default shell interpreter])