science/xtb: Broken on systems where OpenMP isn't enabled

Reported by:	fallout
Approved by:	portmgr (blanket)

science/xtb/Makefile

@@ -9,6 +9,10 @@ COMMENT=	Semiempirical Extended Tight-Binding Program Package
 LICENSE=	GPLv3
 LICENSE_FILE=	${WRKSRC}/COPYING
 
+.if !exists(/usr/include/omp.h)
+BROKEN=		requires OpenMP support that is missing on this architecture
+.endif
+
 USES=		fortran cmake blaslapack:openblas
 
 USE_GITHUB=	yes