build-sys: enable LTO

[masatake]

Close #2885.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (94d7724) 85.36% compared to head (28d7248) 85.36%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3922   +/-   ##
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  Coverage   85.36%   85.36%           
=======================================
  Files         234      234           
  Lines       56123    56125    +2     
=======================================
+ Hits        47907    47909    +2     
  Misses       8216     8216           

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[masatake]

https://stackoverflow.com/questions/40374061/autoconf-recipe-to-use-gcc-ar-and-gcc-ranlib
https://gcc.gnu.org/wiki/LinkTimeOptimizationFAQ#ar.2C_nm_and_ranlib

[leleliu008]

doesn't consider adding a configure option like --enable-lto ？ We could enable LTO by default in release mode. We also allow user to disable it.

[leleliu008]

Why not enable LTO when cross compiling?

[masatake]

@leleliu008 Thank you. I added --enable-lto/--disable-lto.

Why not enable LTO when cross compiling?

I don't know whether my change works well with cross-compiling.
Do you think just removing it?

AS_IF([test "x$cross_compiling" != "xyes" -a "x$enable_lto" != "xno"],[

is enough?

AC_CHECK_PROGS([LTO_AR], [gcc-ar ar]

this may be too simple to make my change work well with cross-compiling.

[leleliu008]

LTO is nothing to do with whether cross compiling or not. I guess it would work if you just removing the cross compiling checking condition, I am on a journey, and didn't bring my laptop, I can't confirm it right now.

[masatake]

LTO is nothing to do with whether cross compiling or not. I guess it would work if you just removing the cross compiling checking condition, I am on a journey, and didn't bring my laptop, I can't confirm it right now.

O.K. I will merge this pull request that disables LTO when cross-compiling.
You can make a pull request to enable LTO + cross-compiling after merging this.