fix warnings: writable string literals

Seen in CLang as:
  conversion from string literal to 'char *' is deprecated [-Wdeprecated-writable-strings]

Replaced `putenv("X=Y")` with `setenv("X", "Y", 1)` as recommended in the
getenv(3) man page, which states "The setenv() function is strongly preferred
to putenv()."
fix warnings: implicit conversion of NULLs

Seen in CLang as:
  warning: implicit conversion of NULL constant to 'bool' [-Wnull-conversion]
fix warnings: writable string literals

Seen in CLang as:
  conversion from string literal to 'char *' is deprecated [-Wdeprecated-writable-strings]

The SDL2_image code is written in C, and C compilers treat string literals as
mutable (non-constant) by default.

C++98 deprecated writable string literals and C++11 made them an error.

There's no reason the upstream C code couldn't also add a couple of `const`
hints for the compiler.
gui panels: removed unused caller_button field

Raised in CLang as:

    warning: private field 'caller_button' is not used [-Wunused-private-field]

The OneMouseButonPanel class declared a private field `caller_button` which
CLang thought was unused. Although caller_button is passed in to
OneMouseButtonPanel constructors and methods, the value is used immediately
(not stored in a field).
Fix for Windows (setenv not available; _putenv preferred over putenv for Windows).
new sound: "we've nuked them!"

I recorded this with the original batch but accidentally left it out.

I license this sample the same as the others (CC0).
Remove code/references to Qt version, no longer supported.
Fix not being able to nuke sectors! (This was a bug introduced since last version 0.26.)
Merge remote-tracking branch 'rhunter/proposed_updates'
Fix assignment of new sample.
Add period character to font.
Support for apostrophes and exclamation marks in fonts!
Reduce volume of explosion so "we've nuked them" speech sample can be heard.
FIXED   If game is already paused, the game going to background shouldn't
        unpause it.
Update credits, history.
UPDATED Improved graphics: unarmed man and caveman.
Improvements to defender graphics - make the player colour more subtle.
Pause music when pausing - important for Android so music pauses when app in background, but useful for other platforms too.
Remove old commented out code.
FIXED   Quitting game now works when screen is fading in.