configure: error: mod_so has been requested but cannot be built on your system

openwrt 交叉编译 apache ，
configure 添加 --enable-so 选项后报错信息如下：

configure: error: mod_so has been requested but cannot be built on your system

原因分析 ：
在编译目录查找打印上面错误的位置，grep “mod_so has been requested but cannot be built on your system” configure -n

httpd-2.4.28# grep "mod_so has been requested but cannot be built on your system" configure -n
13655:            as_fn_error $? "mod_so has been requested but cannot be built on your system" "$LINENO" 5

在 configure 文件的 第 13655 行：查看其上下文：

cat confdefs.h - <<_ACEOF >conftest.$ac_ext
/* end confdefs.h.  */

#include <apr.h>
#if APR_HAS_DSO
YES_IS_DEFINED
#endif

_ACEOF
if (eval "$ac_cpp conftest.$ac_ext") 2>&5 |
  $EGREP "YES_IS_DEFINED" >/dev/null 2>&1; then :
  ac_cv_define_APR_HAS_DSO=yes
else
  ac_cv_define_APR_HAS_DSO=no
fi
rm -f conftest*

CPPFLAGS=$apr_old_cppflags


case "x$enable_so" in
    "xyes")
        if test $ac_cv_define_APR_HAS_DSO = "no"; then
            as_fn_error $? "mod_so has been requested but cannot be built on your system" "$LINENO" 5
        fi
        ;;

可知 configure 脚本检查了 APR_HAS_DSO 宏定义，没有定义APR_HAS_DSO 时就会报以上错误，
APR_HAS_DSO 位于 apr.h 中，来自 libapr 。

检查 libapr 的 编译选项： 位于 openwrt\feeds\packages\libs\apr\Makefile

 CONFIGURE_ARGS += \
        --with-devrandom=/dev/urandom \
        --disable-dso \
        $(call autoconf_bool,CONFIG_IPV6,ipv6)

openwrt 编译 libapr 时 开启了 --disable-dso 导致 APR_HAS_DSO 宏未定义

解决办法：
修改 openwrt\feeds\packages\libs\apr\Makefile ， 把 --disable-dso 选项去掉

 CONFIGURE_ARGS += \
        --with-devrandom=/dev/urandom \       
        $(call autoconf_bool,CONFIG_IPV6,ipv6)

从新编译libapr 后再编译 apache