Enable auto tests under STRICT_ABI if static libs are available.

[iphydf]

STRICT_ABI only breaks linkage if static libraries are not available,
because then we try to link against shared libraries with hidden symbols.

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This change is 

[codecov]

Codecov Report

Merging #1289 into master will decrease coverage by <.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1289     +/-   ##
========================================
- Coverage    83.1%     83%   -0.1%     
========================================
  Files          82      82             
  Lines       14928   14928             
========================================
- Hits        12413   12401     -12     
- Misses       2515    2527     +12

Impacted Files	Coverage Δ
auto_tests/toxav_many_test.c	92.5% <0%> (-4.1%)	⬇️
toxav/msi.c	65% <0%> (-1.7%)	⬇️
toxcore/friend_connection.c	94.9% <0%> (-1.2%)	⬇️
toxav/toxav.c	67.3% <0%> (-0.9%)	⬇️
toxcore/Messenger.c	87% <0%> (ø)	⬆️
toxcore/DHT.c	77.8% <0%> (ø)	⬆️
toxcore/net_crypto.c	95.1% <0%> (+0.1%)	⬆️
toxcore/onion_client.c	96% <0%> (+0.6%)	⬆️
toxcore/LAN_discovery.c	83.9% <0%> (+0.9%)	⬆️

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