Add Abstraction for OpenMP

[RoyiAvital]

Add dedicated option to enable or disable OpenMP.
For instance something like: open_mp("on") or open_mp("off") with "off" being the default.

It will automatically set the correct compilation flags (-fopenmp, /openmp, -qopenmp) and the linking.
Also it will be nice to be able to add advanced mode where the user points to the OpenMP library file (For instance, CLang can use Intel's OpenMP file which helps with compatibility in many cases [Example: MATLAB which uses Intel's OpenMP]).

[waruqi]

I think it would be more suitable to support openmp by means of ʻadd_requires/add_packages/find_packages`.

for example:

Install openmp libraries and enable openmp flags.

add_requires("openmp", {configs = {...}}) 

target("test")
    set_kind("binary")
    add_files("src/*.c")
    add_packages("openmp")

Only find openmp libraries and enable openmp flags

target("test")
    set_kind("binary")
    add_files("src/*.c")
    on_load(function (target)
        target:add(find_packages("openmp"))
    end)

Some references:

• https://gist.github.com/mirkow/e7fefc1200b92e393315213cb086cbb8
• https://github.com/Homebrew/linuxbrew-core/blob/7bad0e1146be7a554cc57ff43f5b811a28140e82/Formula/libomp.rb

[RoyiAvital]

As long as it has its own well documented section I think it is OK.
Just we need to take care of OpenMP as it is widely used.

Thanks.

[waruqi]

Ok, I will arrange time to add it later, please wait some time.

[waruqi]

I have supported it. see #990

for C

add_requires("libomp", {optional = true})
target("hello")
    set_kind("binary")
    add_files("src/*.c")
    add_rules("c.openmp")
    add_packages("libomp")

for C++

add_requires("libomp", {optional = true})
target("hello")
    set_kind("binary")
    add_files("src/*.cpp")
    add_rules("c++.openmp")
    add_packages("libomp")

[waruqi]

some project examples: https://github.com/xmake-io/xmake/tree/dev/tests/projects/openmp