tests: fix spurious failure with Portland Compilers

tests: fix spurious failure with Portland Compilers

* tests/silent-many-generic.test (configure.ac): Be more careful
in the workaround to avoid spurious failures with C++ compilers
containing the string "CC" in their names; this avoids spurious
failures with at least the Sun Studio C++ compiler (when named
"sunCC") and the Portland Group C++ Compiler ("pgCC").
Also handle arguments with whitespace correctly in the 'am--cxx'
temporary wrapper.
* THANKS: Update.

Co-authored-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Copyright-paperwork-exempt: yes