Install project_specific_defines.h if configured (required by config.h)

[brevilo]

Fixes #3471

Description of the Change
Installs project_specific_defines.h in PREFIX/include/boinc if --enable-install-headers is configured. It's included by config.h which in turn gets used when building science apps or screensavers. Right now config.h isn't installed, but that's a separate issue dealt with in #2964.

Release Notes
n/a

[codecov]

Codecov Report

Merging #3493 into master will not change coverage by %.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #3493   +/-   ##
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  Coverage    9.27%   9.27%           
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  Files          36      36           
  Lines        5994    5994           
======================================
  Hits          556     556           
  Misses       5438    5438           

[brevilo]

Hey @AenBleidd or @lfield, could you have a look at this one? Thanks

[AenBleidd]

Ideally, svn_version.h file should be refactored and renamed because now we're using git and it is a little bit confusing. But this is not in the scope of this ticket, so I'm ok.

[brevilo]

Thanks!