build-lldb-mi: Disallow detecting LLVM outside of $LLVM_DIR

Do the same as when cross compiling; disallow detecting dependencies
outside of the given CMAKE_FIND_ROOT_PATH. Don't export LLVM_DIR
as we no longer need to pass it implicitly that way.

This makes the errors clearer, if LLVM isn't found where expected,
instead of misdetecting it from a different installation of LLVM
in the system.

If lldb-mi would try to find other optional dependencies, they would
no longer be found; this was already the case when cross compiling
though.

build-lldb-mi.sh

@@ -65,7 +65,7 @@ else
     esac
 fi
 
-export LLVM_DIR="$PREFIX"
+LLVM_DIR="$PREFIX"
 
 # Try to find/guess the builddir under the llvm buildtree next by.
 # If LLVM was built without LLVM_INSTALL_TOOLCHAIN_ONLY, and the LLVM
@@ -91,7 +91,7 @@ if [ -d "$LLVM_SRC" ]; then
     done
     if [ -n "$DIRS" ]; then
         dir="$(ls -td $DIRS | head -1)"
-        export LLVM_DIR="$LLVM_SRC/$dir"
+        LLVM_DIR="$LLVM_SRC/$dir"
         echo Using $LLVM_DIR as LLVM build dir
         break
     else
@@ -121,14 +121,14 @@ if [ -n "$HOST" ]; then
         exit 1
         ;;
     esac
-
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH=$LLVM_DIR"
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER"
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY"
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=ONLY"
-    CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ONLY"
 fi
 
+CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH=$LLVM_DIR"
+CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER"
+CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY"
+CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=ONLY"
+CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ONLY"
+
 if [ -n "$MACOS_REDIST" ]; then
     : ${MACOS_REDIST_ARCHS:=arm64 x86_64}
     : ${MACOS_REDIST_VERSION:=10.12}