The installer is (once again) running with Admin privilege elevation even for
a Standard Windows user, so it can't determine the underlying Standard user.
Therefore it can't clean up any per-user shortcuts left over for that Standard
user. Moved the kludge to SLVersionChecker.

But since that viewer-manager build depends on being able to read the
application name from build_data.json (to locate the shortcut), add an AppName
key to build_data.json.

instead of a variable of type decltype(expression).

Using SHGetKnownFolderPath(FOLDERID_Fonts) in LLFontGL::getFontPathSystem()
requires new Windows #include files.

A variable with a constructor can't be declared within the braces of a switch
statement, even outside any of its case clauses.

Use LLStringUtil::getenv() or getoptenv() whenever we fetch a string that will
be used as a pathname.

Use LLFile::tmpdir() instead of getenv("TEMP").

As an added extra-special bonus, finally clean up $TMP/llcontrol-test-zzzzzz
directories that have been accumulating every time we run a local build!

Move Windows-flavored llstring_getoptenv() to Windows-specific section of
llstring.cpp.

boost::optional type must be stated explicitly to initialize with a value.

On platforms where llwchar is the same as wchar_t, LLWString is the same as
std::wstring, so ll_convert specializations for std::wstring would duplicate
those for LLWString. Defend against that.

The compilers we use don't like 'return condition? { expr } : {}', in which we
hope to construct and return an instance of the declared return type without
having to restate the type. It works to use an explicit 'if' statement.

Add ll_convert<TO, FROM> template, used as (e.g.):
ll_convert<std::string>(value_of_some_other_string_type);
There is no generic template implementation -- the template exists solely to
provide generic aliases for a bewildering family of llstring.h string-
conversion functions with highly-specific names. There's a generic
implementation, though, for the degenerate case where FROM and TO are
identical.

Add ll_convert<> specialization aliases for most of the string-conversion
functions declared in llstring.h, including the Windows-specific ones
involving llutf16string and std::wstring.

Add a mini-lecture in llstring.h about appropriate use of string types on
Windows.

Add LL_WCHAR_T_NATIVE llpreprocessor.h macro so we can detect whether to
provide separate conversions for llutf16string and std::wstring, or whether
those would collide because the types are identical.

Add inline ll_convert_wide_to_string(const std::wstring&) overloads so caller
isn't required to call arg.c_str(), which naturally permits an ll_convert
alias.

Add ll_convert_wide_to_wstring(), ll_convert_wstring_to_wide() as placeholders
for converting between Windows std::wstring and Linden LLWString, with
corresponding ll_convert aliases. We don't yet have library code to perform
such conversions officially; for now, just copy characters.

Add LLStringUtil::getenv(key) and getoptenv(key) functions. The latter returns
boost::optional<string_type> in case the caller needs to detect absence of a
given environment variable rather than simply accepting a default value.
Naturally getenv(), which accepts a default, is implemented using getoptenv().
getoptenv(), in turn, is implemented using an underlying llstring_getoptenv().

On Windows, llstring_getoptenv() returns boost::optional<std::wstring> (based
on GetEnvironmentVariableW()), whereas elsewhere, llstring_getoptenv() returns
boost::optional<std::string> (based on classic Posix getenv()).

The beauty of generic ll_convert is that the portable LLStringUtilBase<T>::
getoptenv() template can call the platform-specific llstring_getoptenv() and
transparently perform whatever conversion is necessary to return the desired
string_type.

Add windows_message<T>(error) template, with an overload that implicitly calls
GetLastError(). We provide a single concrete windows_message<std::wstring>()
implementation because that's what we get from Windows FormatMessageW() --
everything else is a generic conversion to the desired target string type.

This obviates llprocess.cpp's previous WindowsErrorString() implementation --
reimplement using windows_message<std::string>().

Instead of returning a wchar_t* and requiring the caller to delete it later,
return a std::basic_string<wchar_t> that's self-cleaning. If the caller wants
a wchar_t*, s/he can call c_str() on the returned string.

Default the code_page parameter to CP_UTF8, since we try to be really
consistent about using UTF-8 encoding for all our internal std::strings.

The previous build declared a static std::ofstream; but the code that
determines the pathname for the log file is called so early that static
objects have not yet been constructed. Declare a pointer instead, and
instantiate it on demand.