There are three changes here.
The first is the most important.

Change 1:  Thank you @cary-ilm
* Rename src/core/common to src/core/openjph

OpenJPH headers are included in application code via `#include
<openjph/ojph_version.h>`. The headers are expected to be in a folder
named "openjph". The cmake configuration places them there in the
installation step.

However, if OpenJPH is incorporated into an application via cmake's
`add_subdirectory`, there is no installation step, so there is no
"openjph" folder, leading the `#include <openjph/ojph_version.h>` to
fail.

Renaming the "common" directory to "openjph" resolves the build issue,
since the headers then live inside the source tree in a directory with
same name as the installation. The use of the "common" directory name
is entirely internal to the OpenJPH build, it has no impact on the
installation. The name should be arbitrary, so there should be no
downside to renaming it this way.

Signed-off-by: Cary Phillips <cary@ilm.com>

* This fixes PR compilation

---------

Signed-off-by: Cary Phillips <cary@ilm.com>
Co-authored-by: Cary Phillips <cary@ilm.com>


Change 2:
Message type constants are not descriptive enough, and they maybe inadvertently replaced by a preprocessor macro.
They are:
```
enum OJPH_MSG_LEVEL : int
  {
    ALL_MSG = 0,  // uninitialized or print all message
    INFO = 1,     // info message
    WARN = 2,     // warning message
    ERROR = 3,    // error message (the highest severity)
    NO_MSG = 4,   // no message (higher severity for message printing only)
  };
```
They were replaced with 
```
  enum OJPH_MSG_LEVEL : int
  {
    OJPH_MSG_ALL_MSG = 0,  // uninitialized or print all message
    OJPH_MSG_INFO = 1,     // info message
    OJPH_MSG_WARN = 2,     // warning message
    OJPH_MSG_ERROR = 3,    // error message (the highest severity)
    OJPH_MSG_NO_MSG = 4,   // no message (higher severity for message printing only)
  };
```
which is less like to have the identified issues.


Change 3:
Addresses the problem identified in @clshortfuse in issue #235