CMake MSVC Parallelizm issue: Don't build adig and ahost simultaneously

Make ahost a dependency of adig to prevent issues with them both
referencing ares_strcasecmp.c and ares_getopt.c.  This appears to be
a bug in the Cmake generator for MSVC project files.

Fixes #796
Fix By: Brad House (@bradh352)

src/tools/CMakeLists.txt

@@ -28,6 +28,9 @@ IF (CARES_BUILD_TOOLS)
 
 	# Build adig
 	ADD_EXECUTABLE (adig adig.c ${SAMPLESOURCES})
+	# Don't build adig and ahost in parallel.  This is to prevent a Windows MSVC
+	# build error due to them both using the same source files.
+	ADD_DEPENDENCIES(adig ahost)
 	TARGET_INCLUDE_DIRECTORIES (adig
 		PUBLIC "$<BUILD_INTERFACE:${PROJECT_BINARY_DIR}>"
 		       "$<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}>"