Specify the default VOL connector to use with an environment variable.

This implicitly adds support for changing the VOL connector for command-line
tools or any application linked with the library.

Also, add 'make check-vol' support for all directories, clearing up necessary
issues in testing scripts, etc.

fortran/Makefile.am

@@ -30,6 +30,12 @@ endif
 ## Only recurse into subdirectories if HDF5 is configured to use Fortran.
 if BUILD_FORTRAN_CONDITIONAL
    SUBDIRS=src test $(TESTPARALLEL_DIR)
+
+# Test with just the native connector, with a single pass-through connector
+# and with a doubly-stacked pass-through.
+VOL_LIST = native "pass_through under_vol=0;under_info={}" \
+           "pass_through under_vol=505;under_info={under_vol=0;under_info={}}"
+
 endif
 
 # All directories that have Makefiles