Stop CMake when system library was not found.

scripts/cmake/ExternalProjectBoost.cmake

@@ -19,6 +19,9 @@ if(NOT Boost_INCLUDE_DIRS)
 	endif()
 	if(OGS_LIB_BOOST STREQUAL "System")
 		find_package(Boost ${OGS_BOOST_VERSION} REQUIRED)
+		if(NOT Boost_FOUND)
+			message(FATAL_ERROR "Aborting CMake because system Boost was not found!")
+		endif()
 	elseif(OGS_LIB_BOOST STREQUAL "Default")
 		find_package(Boost ${OGS_BOOST_VERSION})
 	endif()

scripts/cmake/ExternalProjectEigen.cmake

 if(OGS_LIB_EIGEN STREQUAL "System")
 	find_package(Eigen3 3.2.5 REQUIRED)
+	if(NOT EIGEN3_FOUND)
+		message(FATAL_ERROR "Aborting CMake because system Eigen was not found!")
+	endif()
 elseif(OGS_LIB_EIGEN STREQUAL "Default")
 	find_package(Eigen3 3.2.5)
 endif()

scripts/cmake/ExternalProjectVtk.cmake

@@ -14,6 +14,9 @@ endif()
 
 if(OGS_LIB_VTK STREQUAL "System")
 	find_package(VTK 6.1 COMPONENTS ${VTK_MODULES} NO_MODULE REQUIRED)
+	if(NOT VTK_FOUND)
+		message(FATAL_ERROR "Aborting CMake because system VTK was not found!")
+	endif()
 elseif(OGS_LIB_VTK STREQUAL "Default" OR DEFINED VTK_DIR)
 	find_package(VTK 6.1 COMPONENTS ${VTK_MODULES} NO_MODULE QUIET)
 endif()