[BUGFIX] Fix building when USE_BLAS option is not provided (#19832)

* Fix building when USE_BLAS option is not provided

* Fix the check for BLAS libraries in specified order priority

Author: akarbown

cmake/ChooseBlas.cmake

@@ -17,15 +17,17 @@
 
 set(BLAS "Open" CACHE STRING "Selected BLAS library")
 set_property(CACHE BLAS PROPERTY STRINGS "Atlas;Open;MKL")
+# ---[ Root folders
+set(INTEL_HOME_ROOT "$ENV{HOME}/intel" CACHE PATH "Folder contains user-installed intel libs")
+set(INTEL_OPT_ROOT "/opt/intel" CACHE PATH "Folder contains root-installed intel libs")
 
 if(DEFINED USE_BLAS)
   set(BLAS "${USE_BLAS}")
 else()
-  if(USE_MKL_IF_AVAILABLE)
-    if(NOT MKL_FOUND)
-      find_package(MKL)
-    endif()
-    if(MKL_FOUND)
+  if (USE_MKL_IF_AVAILABLE)
+    # Setting up BLAS_mkl_MKLROOT for non-Ubuntu 20.04 OSes
+    find_path(BLAS_mkl_MKLROOT mkl PATHS $ENV{MKLROOT} ${INTEL_HOME_ROOT} ${INTEL_OPT_ROOT})
+    if(NOT BLAS_mkl_MKLROOT STREQUAL "BLAS_mkl_MKLROOT-NOTFOUND")
       set(BLAS "MKL")
     endif()
   endif()
@@ -121,10 +123,6 @@ set(FORTRAN_DIR \\\"\$\{CMAKE_Fortran_IMPLICIT_LINK_DIRECTORIES\}\\\")
     endif()
   endif()
 elseif(BLAS STREQUAL "MKL" OR BLAS STREQUAL "mkl")
-  # ---[ Root folders
-  set(INTEL_HOME_ROOT "$ENV{HOME}/intel" CACHE PATH "Folder contains user-installed intel libs")
-  set(INTEL_OPT_ROOT "/opt/intel" CACHE PATH "Folder contains root-installed intel libs")
-
   # ---[ MKL Options
   if(UNIX)
     # Single dynamic library interface leads to conflicts between intel omp and llvm omp