[squid-dev] RFC: policy change for header #includes

[Amos Jeffries]

Current Policy 
<https://wiki.squid-cache.org/DeveloperResources/SquidCodingGuidelines#file-include-guidelines>:
"
  4. system C headers (with a .h suffix):
     * mandatory HAVE_FOO_H wrapper
"


I propose using the C++17 "__has_include(<foo.h>)" instead of HAVE_FOO_H 
whenever we can. Which is:
  * all .cc and .cci files
  * any .h files in the src/ and tools/ areas


Pros:
  * less configure.ac logic, smaller build logs
  * removes need to do a full bootstrap + re-configure when a 
third-party library or system header is added/remove/changed
  * conversion can be semi-scripted

Cons AFAIK are all about how it cannot be a blanket requirement like 
HAVE_FOO_H is:
  * sometimes headers need to be forbidden include by ./configure logic 
checks
  * C language does not support the __has_include


Cheers
Amos