AX_BOOST_SYSTEM failure scenario

[Awemany]

Hi,

this is a bug report regarding the ax_boost_system.m4 macro of sorts, concerning commit with ID d48ec2210047bbf17702700811c3f16a5369e06c which causes a regression for me. This commit changed the macro so that the CXXFLAGS environment variable is reset to be empty when testing for libboost-system.

While configuring an application to compile with clang-3.8 and setting CXXFLAGS="-std=c++14" to avoid errors about undefined __float128 data types, detection of libboost-system fails with just such an error because it deliberately ignores this compiler flag set to get the application to compile properly. Removing the line that resets CXXFLAGS makes the macro work again.

I understand the issue of the original committer to be able to test for libboost-system while "-Werror" is defined. It seems like a "-std=c++14" to make configure compatible with clang-3.8 should work equally well, however - and it seems like the macros should work in the environment that is defined when invoking ./configure.

Not being an autoconf macro maintainer, I do not know of the proper way to fix this while also caring about the "-Werror" issue, though still want to hereby report it.

Thanks.