also search among COVISEDIR siblings for extern_libs

$COVISEDIR/extern_libs is not supported by cmake:
CMake Error in CMakeLists.txt:
  Target "Fonts" INTERFACE_INCLUDE_DIRECTORIES property contains path:

    "....."

  which is prefixed in the source directory.

Author: aumuell

.covise.sh

@@ -75,6 +75,12 @@ if [ -z "$EXTERNLIBS" ]; then
      extlibs="${COVISEDIR}/extern_libs/${BASEARCH}"
   fi
 
+  if [ ! -d "$extlibs" ]; then
+     extlibs="${COVISEDIR}/../extern_libs/${ARCHSUFFIX}"
+     if [ ! -d "$extlibs" ]; then
+        extlibs="${COVISEDIR}/../extern_libs/${BASEARCH}"
+     fi
+  fi
   if [ ! -d "$extlibs" ]; then
      extlibs="/data/extern_libs/$ARCHSUFFIX"
      if [ ! -d "$extlibs" ]; then