Eliminate duplicate copies of jpeg_nbits_table

ef9a4e05ba (libjpeg-turbo 1.4.x), which
was based on
https://bug815473.bmoattachments.org/attachment.cgi?id=692126
(https://bugzilla.mozilla.org/show_bug.cgi?id=815473), modified the C
baseline Huffman encoder so that it precomputes jpeg_nbits_table, in
order to facilitate sharing the table among multiple processes.
However, libjpeg-turbo never shared the table, and because the table was
implemented as a static array, f3a8684cd1
(libjpeg-turbo 1.5.x) and 37bae1a0e9
(libjpeg-turbo 2.0.x) each introduced a duplicate copy of the table for
(respectively) the SSE2 baseline Huffman encoder and the C progressive
Huffman encoder.

This commit does the following:
- Move the duplicated code in jchuff.c and jcphuff.c, originally
  introduced in 0cfc4c17b7 and
  37bae1a0e9, into a header
  (jpeg_nbits.h).
- Credit the co-author of 0cfc4c17b7.
  (Refer to https://sourceforge.net/p/libjpeg-turbo/patches/57).
- Modify the SSE2 baseline Huffman encoder so that the C Huffman
  encoders can share its definition of jpeg_nbits_table.
- Move the definition of jpeg_nbits_table into a C source file
  (jpeg_nbits.c) rather than a header, and define the table only if
  USE_CLZ_INTRINSIC is undefined and the SSE2 baseline Huffman encoder
  will not be built.
- Apply hidden symbol visibility to the shared definition of
  jpeg_nbits_table, if the compiler supports the necessary attribute.
  (In practice, only Visual C++ doesn't.)

Closes #114

See also:
https://bugzilla.mozilla.org/show_bug.cgi?id=1501523

jconfigint.h.in

@@ -1,6 +1,9 @@
 /* libjpeg-turbo build number */
 #define BUILD  "@BUILD@"
 
+/* How to hide global symbols. */
+#define HIDDEN  @HIDDEN@
+
 /* Compiler's inline keyword */
 #undef inline