MSVC: Eliminate C4996 warnings in API libs

The primary purpose of this is to encourage adoption of libjpeg-turbo in
downstream Windows projects that forbid the use of "deprecated"
functions.  libjpeg-turbo's usage of those functions was not actually
unsafe, because:

- libjpeg-turbo always checks the return value of fopen() and ensures
  that a NULL filename can never be passed to it.

- libjpeg-turbo always checks the return value of getenv() and never
  passes a NULL argument to it.

- The sprintf() calls in format_message() (jerror.c) could never
  overflow the destination string buffer or leave it unterminated as
  long as the buffer was at least JMSG_LENGTH_MAX bytes in length, as
  instructed. (Regardless, this commit replaces those calls with
  snprintf() calls.)

- libjpeg-turbo never uses sscanf() to read strings or multi-byte
  character arrays.

- Because of b7d6e84d6a, wrjpgcom
  explicitly checks the bounds of the source and destination strings
  before calling strcat() and strcpy().

- libjpeg-turbo always ensures that the destination string is
  terminated when using strncpy().
  (548490fe5e made this explicit.)

Regarding thread safety:

Technically speaking, getenv() is not thread-safe, because the returned
pointer may be invalidated if another thread sets the same environment
variable between the time that the first thread calls getenv() and the
time that that thread uses the return value.  In practice, however, this
could only occur with libjpeg-turbo if:

(1) A multithreaded calling application used the deprecated and
undocumented TJFLAG_FORCEMMX/TJFLAG_FORCESSE/TJFLAG_FORCESSE2 flags in
the TurboJPEG API or set one of the corresponding environment variables
(which are only intended for testing purposes.)  Since the TurboJPEG API
library only ever passed string constants to putenv(), the only inherent
risk (i.e. the only risk introduced by the library and not the calling
application) was that the SIMD extensions may have read an incorrect
value from one of the aforementioned environment variables.

or

(2) A multithreaded calling application modified the value of the
JPEGMEM environment variable in one thread while another thread was
reading the value of that environment variable (in the body of
jpeg_create_compress() or jpeg_create_decompress().)  Given that the
libjpeg API provides a thread-safe way for applications to modify the
default memory limit without using the JPEGMEM environment variable,
direct modification of that environment variable by calling applications
is not supported.

Microsoft's implementation of getenv_s() does not claim to be
thread-safe either, so this commit uses getenv_s() solely to mollify
Visual Studio.  New inline functions and macros (GETENV_S() and
PUTENV_S) wrap getenv_s()/_putenv_s() when building for Visual Studio
and getenv()/setenv() otherwise, but GETENV_S()/PUTENV_S() provide no
advantages over getenv()/setenv() other than parameter validation.  They
are implemented solely for convenience.

Technically speaking, strerror() is not thread-safe, because the
returned pointer may be invalidated if another thread changes the locale
and/or calls strerror() between the time that the first thread calls
strerror() and the time that that thread uses the return value.  In
practice, however, this could only occur with libjpeg-turbo if a
multithreaded calling application encountered a file I/O error in
tjLoadImage() or tjSaveImage().  Since both of those functions
immediately copy the string returned from strerror() into a thread-local
buffer, the risk is minimal, and the worst case would involve an
incorrect error string being reported to the calling application.
Regardless, this commit uses strerror_s() in the TurboJPEG API library
when building for Visual Studio.  Note that strerror_r() could have been
used on Un*x systems, but it would have been necessary to handle both
the POSIX and GNU implementations of that function and perform
widespread compatibility testing.  Such is left as an exercise for
another day.

Fixes #568

tjutil.h

@@ -27,11 +27,6 @@
  */
 
 #ifdef _WIN32
-#ifndef __MINGW32__
-#include <stdio.h>
-#define snprintf(str, n, format, ...) \
-  _snprintf_s(str, n, _TRUNCATE, format, ##__VA_ARGS__)
-#endif
 #define strcasecmp  stricmp
 #define strncasecmp  strnicmp
 #endif