Support set global policies in xmake.lua

[waruqi]

Apis

• set_policy

v2.3.4

• set_policy("check.auto_ignore_flags", false), @see Add check flags failed tips for add_cxflags, .. #782
• set_policy("check.auto_map_flags", false)
• set_policy("build.across_targets_in_parallel", false)

[waruqi]

I have finished it and we can run xmake l core.project.policy.policies to show all builtin policies.

$ xmake l core.project.policy.policies
{ 
  "check.auto_map_flags" = { 
    type = "boolean",
    description = "Enable map gcc flags to the current compiler and linker automatically.",
    default = true 
  },
  "build.across_targets_in_parallel" = { 
    type = "boolean",
    description = "Enable compile the source files for each target in parallel.",
    default = true 
  },
  "check.auto_ignore_flags" = { 
    type = "boolean",
    description = "Enable check and ignore unsupported flags automatically.",
    default = true 
  } 
}