Add conan support

[AndreyMlashkin]

Conan is a Package and Dependency Manager. It is a convenient tool to manage multiple dependencies.
It would be great to have a possibility to use DiligentEngine as a conan recipe.
conan-io/conan-center-index#1533

[TheMostDiligent]

@AndreyMlashkin FYI, I've made a set of changes to make dependencies more flexible, take a look at #40.
This does not change the default build system, but allows all third-party libraries to be defined outside of Diligent.

[TheMostDiligent]

Ideally, there will be no need to use patches as CMake should be able to pick up any existing third-party libs and use them instead of submodules.

I am almost sure that everything can now be achieved by just defining aliases where necessary in the top-level CMake, e.g (assuming that non-namespace targets are not defined - I don't know if they actually are).

find_package(glslang REQUIRED CONFIG)
add_library(glslang ALIAS glslang::glslang)
add_library(OGLCompiler ALIAS glslang::OGLCompiler)


add_subdirectory(source_subfolder)

[AndreyMlashkin]

@TheMostDiligent, in v2.5.2 it works really good for all libraries but glslang, there we have

CMake Error at source_subfolder/ThirdParty/CMakeLists.txt:68 (add_subdirectory):

[AndreyMlashkin]

-    if (NOT ${DILIGENT_NO_GLSLANG} AND (NOT TARGET glslang))
+    if (NOT ${DILIGENT_NO_GLSLANG} AND (NOT TARGET glslang::glslang))

This fixes my problem

[TheMostDiligent]

glslang's CMake file does not add namespace unless the targets are installed, so TARGET glslang::glslang will not work for the default build.
My suggestion is to add backwards aliases before including Diligent source:

find_package(glslang REQUIRED CONFIG)
add_library(glslang ALIAS glslang::glslang)
add_library(OGLCompiler ALIAS glslang::OGLCompiler)
# And similar for other required targets

add_subdirectory(source_subfolder)