Change library version parsing to use shell components

[Lastique]

This fixes the build failure on Ubuntu 20.10, where the previous parsing would result in incorrect version components that included
the "#define" prefix.

For example:

MAJOR_VERSION=#define 2
MINOR_VERSION=#define 10
BUILD_NUMBER=#define 9
BUILD_TYPE=#define .

[willamowius]

When I compiled PTLib on Ubuntu 20.10 with the original configure I didn't see any errors.

Is there anything I need to do to make it happen ?

[Lastique]

I didn't do anything special, except that I'm building a .deb package. The build succeeded for Ubuntu 20.04 and Debian 9 & 10, but failed for Ubuntu 20.10 because of these prefixes. I'm not sure what is different in 20.10.

PS: I'm about to push an update to this PR, don't merge just yet.

[Lastique]

Done. I had to add extra space trimming because libh323plus has multiple spaces between the macro names and the values.

[Lastique]

As to why this is needed, I really don't know. Maybe there's some sort of macro or environment variable that is defined when the package is being built, and that affects make behavior. I tried to debug this, but eventually decided to use shell commands as a more reliable solution. Similar parsers are already used in configure scripts anyway.