Provide a default lib location when detecting C++11 ABI

Useful for pkgconfig without -L, e.g. Alpine Linux
See also commit 07d66da

binding.gyp

@@ -107,7 +107,7 @@
         ],
         'defines': [
           # Inspect libvips-cpp.so to determine which C++11 ABI version was used and set _GLIBCXX_USE_CXX11_ABI accordingly. This is quite horrible.
-          '_GLIBCXX_USE_CXX11_ABI=<!(if readelf -Ws "$(PKG_CONFIG_PATH="<(pkg_config_path)" pkg-config --libs-only-L vips-cpp | cut -c 3-)/libvips-cpp.so" | c++filt | grep -qF __cxx11;then echo "1";else echo "0";fi)'
+          '_GLIBCXX_USE_CXX11_ABI=<!(if readelf -Ws "$(PKG_CONFIG_PATH="<(pkg_config_path)" pkg-config --libs-only-L vips-cpp | cut -c 3- | sed -e "s/^$/\/usr\/lib/")/libvips-cpp.so" | c++filt | grep -qF __cxx11;then echo "1";else echo "0";fi)'
         ]
       }, {
         # Attempt to download pre-built libvips and install locally within node_modules