make.inc.gfortran: define a default value for DOCSDIR

[svillemot]

The same has already be done in make.inc.nagfor.

Actually it could make sense to define DOCSDIR in all the make.inc.*, for consistency.

[codecov]

Codecov Report

Merging #618 (a5ea879) into master (c8cb980) will not change coverage.
The diff coverage is n/a.

❗ Current head a5ea879 differs from pull request most recent head c657043. Consider uploading reports for the commit c657043 to get more accurate results

@@           Coverage Diff           @@
##           master     #618   +/-   ##
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  Coverage    0.00%    0.00%           
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  Files        1894     1894           
  Lines      184021   184021           
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  Misses     184021   184021           

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