The brackets in the config test aren't properly escaped, and thus
don't end up in the final configure script as intended.

When building on a system where the system default compiler is clang,
clang can fail due to the main signature
"int main(int argc, char *argv)" being invalid,
which makes configure believe that the system compiler isn't
ANSI compliant, and it tries to invoke (broken) ANSI2KNR code.
Signed-off-by: Martin Storsjö <martin@martin.st>