compilation failure on ppc with -DCMAKE_BUILD_TYPE=Debug

if ${CRC32_LIBRARY} target has no COMPILE_FLAGS yet,
GET_TARGET_PROPERTY returns COMPILE_FLAGS-NOTFOUND, which
doesn't work very well when it's later fed back into COMPILE_FLAGS.

GET_PROPERTY() returns an empty string in this case.

extra/crc32-vpmsum/CMakeLists.txt

@@ -2,7 +2,7 @@ ADD_CONVENIENCE_LIBRARY(${CRC32_LIBRARY} $<TARGET_OBJECTS:crc32c> $<TARGET_OBJEC
 ADD_LIBRARY(crc32c OBJECT vec_crc32.c)
 ADD_LIBRARY(crc32ieee OBJECT vec_crc32.c)
 
-GET_TARGET_PROPERTY(CFLAGS_CRC32_VPMSUM ${CRC32_LIBRARY} COMPILE_FLAGS)
+GET_PROPERTY(CFLAGS_CRC32_VPMSUM TARGET ${CRC32_LIBRARY} PROPERTY COMPILE_FLAGS)
 SET_TARGET_PROPERTIES(crc32c crc32ieee PROPERTIES  COMPILE_FLAGS "${CFLAGS_CRC32_VPMSUM} -maltivec -mvsx -mpower8-vector -mcrypto -mpower8-vector")
 SET_TARGET_PROPERTIES(crc32ieee PROPERTIES COMPILE_DEFINITIONS "CRC32_FUNCTION=crc32ieee_vpmsum;CRC32_CONSTANTS_HEADER=\"crc32ieee_constants.h\"")
 SET_TARGET_PROPERTIES(crc32c PROPERTIES COMPILE_DEFINITIONS "CRC32_FUNCTION=crc32c_vpmsum;CRC32_CONSTANTS_HEADER=\"crc32c_constants.h\"")