bug#27346: module tests fail to compile with gcc 4.8.5

[Glenn Morris]

Package: emacs
Version: 26.0.50

Current master on rhel7.3.
gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-11)

After building with-modules:
    cd test/data/emacs-module
    make
    
    gcc -std=gnu11 -I../../../src -fPIC -g3 -O2 -c -o mod-test.o mod-test.c
    In file included from mod-test.c:23:0:
    ../../../src/emacs-module.h:38:47: error: missing binary operator before 
token "("
     #if defined __has_attribute && __has_attribute(__nonnull__)
                                                   ^
    make: *** [mod-test.o] Error 1