x11-toolkits/kf5-kirigami2: fix build without libomp

CMake Error at /usr/local/share/cmake/Modules/FindOpenMP.cmake:261 (try_compile):
  Failed to generate test project build system.
Call Stack (most recent call first):
  /usr/local/share/cmake/Modules/FindOpenMP.cmake:537 (_OPENMP_GET_FLAGS)
  CMakeLists.txt:122 (find_package)

x11-toolkits/kf5-kirigami2/Makefile

@@ -5,7 +5,12 @@ CATEGORIES=	x11-toolkits kde kde-frameworks
 MAINTAINER=	kde@FreeBSD.org
 COMMENT=	QtQuick based components set
 
-USES=		cmake compiler:c++11-lib gettext kde:5 qt:5 tar:xz
+USES=		cmake gettext kde:5 qt:5 tar:xz
+.if !exists(/usr/include/omp.h)
+USES+=		compiler:gcc-c++11-lib
+.else
+USES+=		compiler:c++11-lib
+.endif
 USE_KDE=	kdeclarative \
 		ecm:build
 USE_QT=		core concurrent dbus declarative graphicaleffects gui \