Update to the latest ax_lua.m4

[morrone]

The older version of ax_lua.m4 that we used was failing to
abort when AX_LUA_HEADERS failed to find headers. That is
no longer a problem in the new ax_lua.m4.

Fixes #1270

[codecov-io]

Codecov Report

Merging #1275 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1275      +/-   ##
==========================================
- Coverage   77.88%   77.87%   -0.02%     
==========================================
  Files         154      154              
  Lines       29124    29124              
==========================================
- Hits        22684    22680       -4     
- Misses       6440     6444       +4

Impacted Files	Coverage Δ
src/common/libflux/request.c	88.46% <0%> (-1.29%)	⬇️
src/common/libflux/mrpc.c	85.49% <0%> (-1.18%)	⬇️
src/broker/modservice.c	80.58% <0%> (-0.98%)	⬇️
src/common/libkvs/treeobj.c	85.82% <0%> (-0.39%)	⬇️
src/broker/module.c	83.79% <0%> (-0.28%)	⬇️
src/common/libflux/handle.c	83.66% <0%> (-0.25%)	⬇️
src/common/libflux/message.c	81.36% <0%> (-0.24%)	⬇️
src/common/libkvs/kvs.c	64.03% <0%> (+0.24%)	⬆️
src/modules/kvs/kvs.c	62.62% <0%> (+0.25%)	⬆️
src/broker/overlay.c	74.2% <0%> (+0.31%)	⬆️
... and 1 more

[coveralls]

Coverage decreased (-0.02%) to 78.493% when pulling 4a664e6 on morrone:ax_lua_update into d7300ec on flux-framework:master.

[grondo]

This looks good to me. Thanks for tracking that and running down the fix.