[cmake] fix typo / rewrite

Co-authored-by: Stephan Hageboeck <[email protected]>

Author: ferdymercury

cmake/modules/SearchInstalledSoftware.cmake

@@ -643,7 +643,7 @@ if(mathmore OR builtin_gsl OR (tmva-cpu AND use_gsl_cblas))
     set(GSL_INCLUDE_DIR ${CMAKE_BINARY_DIR}/GSL-prefix/src/GSL-build)
     set(GSL_FOUND ON)
     if(NOT mathmore)
-      message(FATAL_ERROR "builtin_gls was enabled, conflict with mathmore=OFF")
+      message(FATAL_ERROR "builtin_gsl requires mathmore=ON")
     endif()
   endif()
 endif()