Fix FindUmfpack when specifying manually the related cmake variables.

cmake/FindUmfpack.cmake

diff --git a/cmake/FindUmfpack.cmake b/cmake/FindUmfpack.cmake
index 3df3a0a..b0cc4e9 100644
--- a/cmake/FindUmfpack.cmake
+++ b/cmake/FindUmfpack.cmake
@@ -16,6 +16,8 @@
   ufsparse
 )
 
+if(NOT UMFPACK_LIBRARIES)
+
 find_library(UMFPACK_LIBRARIES umfpack PATHS $ENV{UMFPACKDIR} ${LIB_INSTALL_DIR})
 
 if(UMFPACK_LIBRARIES)
@@ -43,6 +45,7 @@
 
 endif(UMFPACK_LIBRARIES)
 
+endif()
 
 include(FindPackageHandleStandardArgs)
 find_package_handle_standard_args(UMFPACK DEFAULT_MSG