Add a generic bsd platform for expressions (#1785)

In most places there's a check for either FreeBSD or OpenBSD it should
actually check for BSD in general. This will make some expressions
cleaner and make adding support for other BSD systems easier.

Author: cqundefine

src/vcpkg/platform-expression.cpp

@@ -27,6 +27,7 @@ namespace vcpkg::PlatformExpression
         linux,
         freebsd,
         openbsd,
+        bsd,
         solaris,
         osx,
         uwp,
@@ -62,6 +63,7 @@ namespace vcpkg::PlatformExpression
             {"linux", Identifier::linux},
             {"freebsd", Identifier::freebsd},
             {"openbsd", Identifier::openbsd},
+            {"bsd", Identifier::bsd},
             {"solaris", Identifier::solaris},
             {"osx", Identifier::osx},
             {"uwp", Identifier::uwp},
@@ -568,6 +570,9 @@ namespace vcpkg::PlatformExpression
                         case Identifier::linux: return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "Linux");
                         case Identifier::freebsd: return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "FreeBSD");
                         case Identifier::openbsd: return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "OpenBSD");
+                        case Identifier::bsd:
+                            return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "FreeBSD") ||
+                                   true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "OpenBSD");
                         case Identifier::solaris: return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "SunOS");
                         case Identifier::osx: return true_if_exists_and_equal("VCPKG_CMAKE_SYSTEM_NAME", "Darwin");
                         case Identifier::uwp: