DIsables OpenMP for no obvious reason

[yurivict]

It finds OpenMP and then says that it is disabled, and doesn't say why:

-- The following OPTIONAL packages have been found:

 * EXPAT
 * OpenColorIO
 * Gettext
 * Msgfmt
 * OpenMP

-- The following REQUIRED packages have been found:

 * TIFF
 * PNG
 * Pugixml (required version >= 1.2), Light-weight, simple and fast XML parser, <http://pugixml.org/>
   Used for loading of data/cameras.xml
 * JPEG, free library for handling the JPEG image data format, implements a JPEG codec
   Used for decoding DNG Lossy JPEG compression
 * ZLIB, software library used for data compression
   Used for decoding DNG Deflate compression

-- The following features have been disabled:

 * OpenMP-based threading, used for parallelization of the library

-- The following REQUIRED packages have not been found:

 * Iconv

[aferrero2707]

PhotoFlow uses explicit multi-threading, therefore OpenMP is redundant.