C++ interface: Extend header test to make it usable with clang++.

* On FreeBSD, g++ might also be available under the name g++${VERSION}.

tests/test_hpp.in

@@ -46,7 +46,7 @@ set -e
 if [ x"${BASH_VERSION}" = x ]; then
   exec bash "$0" || { echo 'test skipped: bash not found!' >&2 ; exit 77 ; }
 fi
-if GXX=$(command -v g++ 2>/dev/null) &&
+if GXX=${GXX-$(command -v g++ 2>/dev/null)} &&
     printf '%s\n' '#include <mpi.h>' | \
       "$GXX" -x c++ -S -o /dev/null -I../inst_headers \
          @MPI_C_INCLUDE@ - \
@@ -55,7 +55,11 @@ if GXX=$(command -v g++ 2>/dev/null) &&
          @MPI_C_INCLUDE@ \
          ../inst_headers/yaxt.hpp
 else
-  echo 'test skipped: Could not find the GNU C++ compiler g++' >&2
+  printf '%s\n' \
+    'test skipped: Could not find the GNU C++ compiler g++ or clang++' \
+    '  consider setting environment variable GXX to a command to' \
+    '  execute a compiler supporting common g++ command-line switches,' \
+    '  e.g. GXX=mpic++ ' >&2
   exit 77
 fi
 #