bugfix: ensured the 'pcre_version' symbol is preserved as undefined when PCRE is statically linked.

When using `--with-pcre=...`, NGINX is statically linked against
libpcre.a. Since `pcre_version()` is unused, its symbol is stripped by
the linker. Because lua-resty-core's `resty.core.regex` module needs it,
we here ensure that the symbol is entered as undefined in our final
binary.

This should not be an issue for win32 builds since
`--export-all-symbols` is already set.

Author: thibaultcha

config

@@ -450,5 +450,23 @@ ngx_feature_test='struct sigaction act;
 
 . auto/feature
 
+if [ $PCRE != NO -a $PCRE != YES ]; then
+    # force pcre_version symbol to be undefined when PCRE is statically linked
+
+    ngx_feature="force undefined symbols (--undefined)"
+    ngx_feature_libs="-Wl,--undefined=printf"
+    ngx_feature_name=
+    ngx_feature_run=no
+    ngx_feature_incs="#include <stdio.h>"
+    ngx_feature_path=
+    ngx_feature_test='printf("hello");'
+
+    . auto/feature
+
+    if [ $ngx_found = yes ]; then
+        CORE_LIBS="$CORE_LIBS -Wl,--undefined=pcre_version"
+    fi
+fi
+
 #CFLAGS=$"$CFLAGS -DLUA_DEFAULT_PATH='\"/usr/local/openresty/lualib/?.lua\"'"
 #CFLAGS=$"$CFLAGS -DLUA_DEFAULT_CPATH='\"/usr/local/openresty/lualib/?.so\"'"