Unconditional expansion to TrueType GDEF table can cause problems.

Code was put in tottfgpos.c in routine otf_dumpgdef() to allow
addition of a newly documented fifth subtable type in TrueType
GDEF tables.  Unfortunately, while actually creating the new
subtable data is conditional on whether needed or not, the newer
fifth offset is always unconditionally inserted into the GDEF
header.  This (small) incompatibility can trouble some programs
reading the unconditionally expanded GDEF header.

When warranted, FF will produce a "mark set definitions" subtable.
When it does so it will change the GDEF table version from
0x00010000 to 0x00010002 to signal that this newer subtable has
been added (and as the newer TTF documentation requests).

However, the code always adds the fifth offset word to the table
header, no matter what header version value was used.

The example failure is Microsoft's utility ttfdump dying when
dumping a TTF file's GDEF table, saying
  ; 'GDEF' Table - Glyph Definition
  ;-------------------------------------
  Error: Table "T000c" not referenced. Data Misalignment?

(The mysterious "T000c" is because of the unsignaled fifth offset
word value of null at offset 0x000C from start of GDEF header.)

This result for ttfdump program version
  ; TrueType v1.0 Dump Program - v1.8, Oct 29 2002, rrt, dra, gch, ddb, lcp, pml

With this fix ttfdump now can completely dump the GDEF table,
and can also now dump the following 4 or more tables missing
from previous ttfdump listings.

This new subtable is documented here
    http://www.microsoft.com/typography/otspec/gdef.htm
and which mentions both "four GDEF tables" and "five GDEF tables"
which gives an indication of how hurried the new addition was.
Add new development package reqd to install instructions

Recently the need for library package uthash was added.
The list of packages for the suggested "sudo apt-get install ..."
command needs to have "uthash-dev" added to it.
Merge pull request #1476 from tshinnic/master

Unconditional expansion to TrueType `GDEF` table can cause problems.