BUILD/MINOR: lua: define _GNU_SOURCE for LLONG_MAX

Lua requires LLONG_MAX defined with __USE_ISOC99 which is set by
_GNU_SOURCE, not necessarely defined by default on old compiler/glibc.

  $ make V=1 TARGET=linux-glibc-legacy USE_THREAD= USE_ACCEPT4= USE_PCRE=1 USE_OPENSSL=1 USE_ZLIB=1  USE_LUA=1
  ..
  cc -Iinclude  -O2 -g -Wall -Wextra -Wdeclaration-after-statement -fwrapv  -Wno-strict-aliasing -Wno-unused-label -Wno-sign-compare -Wno-unused-parameter  -Wno-missing-field-initializers              -DUSE_EPOLL  -DUSE_NETFILTER -DUSE_PCRE    -DUSE_POLL       -DUSE_TPROXY -DUSE_LINUX_TPROXY -DUSE_LINUX_SPLICE -DUSE_LIBCRYPT -DUSE_CRYPT_H -DUSE_GETADDRINFO -DUSE_OPENSSL -DUSE_LUA -DUSE_FUTEX   -DUSE_ZLIB  -DUSE_CPU_AFFINITY   -DUSE_DL -DUSE_RT      -DUSE_PRCTL -DUSE_THREAD_DUMP     -I/usr/include/openssl101e/  -DUSE_PCRE -I/usr/include  -DCONFIG_HAPROXY_VERSION=\"2.4-dev5-73246d-83\" -DCONFIG_HAPROXY_DATE=\"2021/01/21\" -c -o src/hlua.o src/hlua.c
  In file included from /usr/local/include/lua.h:15,
                   from /usr/local/include/lauxlib.h:15,
                   from src/hlua.c:16:
  /usr/local/include/luaconf.h:581:2: error: #error "Compiler does not support 'long long'. Use option '-DLUA_32BITS'   or '-DLUA_C89_NUMBERS' (see file 'luaconf.h' for details)"
  ..
  cc -Iinclude  -O2 -g -Wall -Wextra -Wdeclaration-after-statement -fwrapv  -Wno-strict-aliasing -Wno-unused-label -Wno-sign-compare -Wno-unused-parameter  -Wno-missing-field-initializers              -DUSE_EPOLL  -DUSE_NETFILTER -DUSE_PCRE    -DUSE_POLL       -DUSE_TPROXY -DUSE_LINUX_TPROXY -DUSE_LINUX_SPLICE -DUSE_LIBCRYPT -DUSE_CRYPT_H -DUSE_GETADDRINFO -DUSE_OPENSSL -DUSE_LUA -DUSE_FUTEX   -DUSE_ZLIB  -DUSE_CPU_AFFINITY   -DUSE_DL -DUSE_RT      -DUSE_PRCTL -DUSE_THREAD_DUMP     -I/usr/include/openssl101e/  -DUSE_PCRE -I/usr/include  -DCONFIG_HAPROXY_VERSION=\"2.4-dev5-73246d-83\" -DCONFIG_HAPROXY_DATE=\"2021/01/21\" -c -o src/hlua_fcn.o src/hlua_fcn.c
  In file included from /usr/local/include/lua.h:15,
                   from /usr/local/include/lauxlib.h:15,
                   from src/hlua_fcn.c:17:
  /usr/local/include/luaconf.h:581:2: error: #error "Compiler does not support 'long long'. Use option '-DLUA_32BITS'   or '-DLUA_C89_NUMBERS' (see file 'luaconf.h' for details)"
  ..

Cc: Thierry Fournier <tfournier@arpalert.org>
diff --git a/src/hlua.c b/src/hlua.c
index 785a1fa..0de1937 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -10,6 +10,8 @@
  *
  */
 
+#define _GNU_SOURCE
+
 #include <ctype.h>
 #include <setjmp.h>
 
diff --git a/src/hlua_fcn.c b/src/hlua_fcn.c
index 1f66c18..aab8643 100644
--- a/src/hlua_fcn.c
+++ b/src/hlua_fcn.c
@@ -14,6 +14,9 @@
  * in an environment able to catch a longjmp, otherwise a
  * critical error can be raised.
  */
+
+#define _GNU_SOURCE
+
 #include <lauxlib.h>
 #include <lua.h>
 #include <lualib.h>