Wrap CXXFLAGS test with double quotes in configure.ac

[frankdean]

Without the quotes when configuring with CXXFLAGS;

./configure CXXFLAGS='-g -O0' CPPFLAGS='-I$(top_srcdir)'

The test is interpreted as:

test -z -g -O0

resulting in an error amongst the output like:

. . .
checking whether gcc understands -c and -o together... (cached) yes
./configure: line 6360: test: -g: binary operator expected
checking for g++... g++
checking whether the compiler supports GNU C++... yes
. . .

[gansm]

Hi Frank, thanks for your pull request. I added this line on October 15, 2021, which was four years, four months, and two days ago. So your fix is just two days too early 😄. Your patch now clearly prevents the word-splitting problem.

[frankdean]

Hi Markus, it's funny how long things can go unnoticed. I must admit it was a while before I noticed the warning message whizzing up the screen, so I'm not surprised!