configure: Change mistaken += to =

[salahcoronya]

The line

AX_CHECK_LINK_FLAG([[-Wl,--gc-sections]], [strip+="y$strip"])

is wrong. The += should be =. So the subsequent test is never true, and chokes on dash because += is a bashism.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (1b3aab9) 72.43% compared to head (03bc273) 72.43%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #855   +/-   ##
=======================================
  Coverage   72.43%   72.43%           
=======================================
  Files          34       34           
  Lines        9773     9773           
=======================================
  Hits         7079     7079           
  Misses       2694     2694           

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