[Bug other/82708] New: libmpx fails to compile due to PATH_MAX

[keno at juliacomputing dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82708

            Bug ID: 82708
           Summary: libmpx fails to compile due to PATH_MAX
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: keno at juliacomputing dot com
  Target Milestone: ---

In trying to build gcc on musl-based Alpine linux, I encountered the following
error:
```
/src/gcc-7.2.0/libmpx/mpxrt/mpxrt-utils.c:72:23: error: ‘PATH_MAX’ undeclared
here (not in a function); did you mean ‘INT8_MAX’?
 #define MAX_FILE_NAME PATH_MAX
                       ^
/src/gcc-7.2.0/libmpx/mpxrt/mpxrt-utils.c:95:22: note: in expansion of macro
‘MAX_FILE_NAME’
 static char out_name[MAX_FILE_NAME];
                      ^~~~~~~~~~~~~
```
I was confused by this at first because musl defines PATH_MAX in limits.h which
appears to be included from that file, but looking at the `-E` output it turns
out that it uses `include-fixed/limits.h` instead, which doesn't have that
definition. I don't know enough about GCC's build system to determine what's at
fault here (maybe it shouldn't be using that limits.h?)

In searching the internet, there are a number of reports of this error from
various other system configurations as well, so this like a fairly common
problem when building gcc.