FindBLAS: Restore BLAS_FOUND when found using pkgconfig

Refactoring in commit v3.12.0-rc1~92^2 (FindPkgConfig: export the list
of found libraries also as variable, 2018-05-11) dropped use of FPHSA
to set `BLAS_FOUND`.  Set it explicitly instead.

Modules/FindBLAS.cmake

@@ -93,6 +93,7 @@ if(BLA_PREFER_PKGCONFIG)
   find_package(PkgConfig)
   pkg_check_modules(PKGC_BLAS blas)
   if(PKGC_BLAS_FOUND)
+    set(BLAS_FOUND ${PKGC_BLAS_FOUND})
     set(BLAS_LIBRARIES "${PKGC_BLAS_LINK_LIBRARIES}")
     return()
   endif()