buildsys: fix a couple english spelling errors in configure.ac

[ploxiln]

introduced in 1293d03
(rbtree: introduce new ifdef condition HAVE_STATEMENT_EXPRESSION_EXT)

[coveralls]

Coverage remained the same at 86.819% when pulling 34421e9 on ploxiln:configure_typo_fix into aac6193 on universal-ctags:master.

[codecov]

Codecov Report

Merging #2597 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2597   +/-   ##
=======================================
  Coverage   86.71%   86.71%           
=======================================
  Files         183      183           
  Lines       38779    38779           
=======================================
  Hits        33627    33627           
  Misses       5152     5152           

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[masatake]

Thank you!