Support building with system-wide glslang

[fff7d1bc]

Hi,

First, thanks for adding the support for building with system-wide ncnn and webp.

Now I am facing issue when waifu2x-ncnn-vulkan wants a path for GLSL directory that contain *.cmake files. When glslang is installed system wide, they're located in /usr/lib(64)/cmake. To make it work on Gentoo currently we need to append "-DGLSLANG_TARGET_DIR=${EPREFIX}/usr/$(get_libdir)/cmake" to our ebuild[1]

Would it be possible for you to maybe default to system-wide glslang if USE_SYSTEM_NCNN? Or maybe a flag that takes every single dependency (libwebp, glslang, ncnn) from system?

[1] https://gitweb.gentoo.org/repo/gentoo.git/tree/media-gfx/waifu2x-ncnn-vulkan/waifu2x-ncnn-vulkan-20200818.ebuild#n36

[fff7d1bc]

@nihui Friendly bump. :)

[davidebeatrici]

Related: #200