Fix USE_PERL option usage

Once you define the USE_PERL variable it is always set as DEFINED regardless of its value. One should chack for the actual variable value.

CMakeLists.txt

@@ -398,7 +398,7 @@ if (BUILD_SHARED_LIBS AND NOT ${SYMBOLPREFIX}${SYMBOLSUFFIX} STREQUAL "")
     message(STATUS "adding suffix ${SYMBOLSUFFIX} to names of exported symbols in ${OpenBLAS_LIBNAME}")
   endif()
 
-  if (NOT DEFINED USE_PERL)
+  if (NOT USE_PERL)
   add_custom_command(TARGET ${OpenBLAS_LIBNAME}_shared POST_BUILD
     COMMAND  ${PROJECT_SOURCE_DIR}/exports/gensymbol "objcopy" "${ARCH}" "${BU}" "${EXPRECISION_IN}" "${NO_CBLAS_IN}" "${NO_LAPACK_IN}" "${NO_LAPACKE_IN}" "${NEED2UNDERSCORES_IN}" "${ONLY_CBLAS_IN}" \"${SYMBOLPREFIX}\" \"${SYMBOLSUFFIX}\" "${BUILD_LAPACK_DEPRECATED}" "${BUILD_BFLOAT16}" "${BUILD_SINGLE}" "${BUILD_DOUBLE}" "${BUILD_COMPLEX}" "${BUILD_COMPLEX16}" > ${PROJECT_BINARY_DIR}/objcopy.def
     COMMAND objcopy -v --redefine-syms ${PROJECT_BINARY_DIR}/objcopy.def  ${PROJECT_BINARY_DIR}/lib/lib${OpenBLAS_LIBNAME}.so