build: Fix handling of missing BLAS.

[pcanal]

find_package for BLAS set BLAS_LIBRARY to a literal FALSE. As used further in the ROOT cmake files this lead to a link line for a couple of sofie test containing . However the value is not cached and thus after a second configuration the problem went away.

Without this, when BLAS is not installed and with the configuration:

cxx17 asimage builtin_clang builtin_cling builtin_gtest builtin_llvm builtin_lz4 builtin_lzma builtin_nlohmannjson builtin_openui5 builtin_xxhash builtin_zlib builtin_zstd clad dataframe davix gdml geom http imt pyroot roofit root7 rpath runtime_cxxmodules shared spectrum sqlite ssl testsupport tmva tmva-cudnn tmva-pymva tpython vdt x11 xml xrootd

You get a could of failures with the complain about -lFALSE:

FAILED: tmva/pymva/test/TestRModelParserPyTorch 
: && /cvmfs/cms.cern.ch/el9_amd64_gcc11/external/gcc/11.4.1-30ebdc301ebd200f2ae0e3d880258e65/bin/c++ -fdiagnostics-color=always -Wno-implicit-fallthrough -Wno-noexcept-type -pipe  -Wshadow -Wall -W -Woverloaded-virtual -fsigned-char -pthread -O3 -DNDEBUG -rdynamic core/testsupport/CMakeFiles/TestSupport.dir/src/TestSupport.cxx.o tmva/pymva/test/CMakeFiles/TestRModelParserPyTorch.dir/TestRModelParserPyTorch.C.o -o tmva/pymva/test/TestRModelParserPyTorch  -Wl,-rpath,/home/pcanal/root_working/build/master/lib:/cvmfs/cms.cern.ch/el9_amd64_gcc11/external/python3/3.9.14-e432d7f95f9e22c05899c3205f44ed54/lib  lib/libROOTTMVASofie.so  lib/libTMVA.so  /cvmfs/cms.cern.ch/el9_amd64_gcc11/external/python3/3.9.14-e432d7f95f9e22c05899c3205f44ed54/lib/libpython3.9.so  googletest-prefix/src/googletest-build/lib//libgtest_main.a  googletest-prefix/src/googletest-build/lib//libgmock.a  googletest-prefix/src/googletest-build/lib//libgmock_main.a  \
-lFALSE lib/libMinuit.so  lib/libMLP.so  lib/libTreePlayer.so  lib/libGraf3d.so  lib/libTree.so  lib/libGpad.so  lib/libGraf.so  lib/libHist.so  lib/libMatrix.so  lib/libMathCore.so  lib/libXMLIO.so  lib/libImt.so  lib/libMultiProc.so  lib/libNet.so  lib/libRIO.so  lib/libThread.so  lib/libCore.so  googletest-prefix/src/googletest-build/lib//libgtest.a && :
/cvmfs/cms.cern.ch/el9_amd64_gcc11/external/gcc/11.4.1-30ebdc301ebd200f2ae0e3d880258e65/bin/../lib/gcc/x86_64-redhat-linux-gnu/11.4.1/../../../../x86_64-redhat-linux-gnu/bin/ld: 
    cannot find -lFALSE: No such file or directory

[hageboeck]

LGTM!

[hageboeck]

I might add BLAS to the modernisation effort in #8709, but that doesn't need to hold back this PR.