Compile error on CentOs 6

[azilber]

Describe the bug

CC redis-cli.o
In file included from fastlock.h:3,
                 from ae.h:40,
                 from redis-cli.c:63:
fastlock.h:88:15: error: expected declaration specifiers or ‘...’ before ‘__builtin_offsetof’
 static_assert(offsetof(struct fastlock, m_ticket) == 64, "ensure padding is correct");
               ^~~~~~~~
In file included from ae.h:40,
                 from redis-cli.c:63:
fastlock.h:88:58: error: expected declaration specifiers or ‘...’ before string constant
 static_assert(offsetof(struct fastlock, m_ticket) == 64, "ensure padding is correct");
                                                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~
make[1]: *** [Makefile:372: redis-cli.o] Error 1

To Reproduce
Use CentOs 6 and any of the devtoolsets: ( scl -l )
devtoolset-7
devtoolset-8
devtoolset-9

This will provide gcc versions 7.3.1, 8.3.1, and 9.1.1

They all give the error above.

[JohnSully]

This is fixed in change f19b9c1. We were using a C++ concept in a normal C file. Other compiles are more permissive about this.