contrib: sidplay: Do not overwrite CXXFLAGS in configure.in

resid configure has a bug which overwrites CXXFLAGS unconditionally, causing build failures when preset CXXFLAGS are needed.

Reason: In newer autoconf, ac_test_CXXFLAGS does not use "set" anymore, but "y".

Also see https://github.com/NanoComp/meep/issues/1934.

Configure error when building resid for 3.0.21 release:

configure:4692: result: yes
configure:4721: checking size of int
configure:4727: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -o conftest -g -Wall -O2 -funroll-loops -fomit-frame-pointer -fno-exce
ptions  -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -mmacosx-version-min=10.7 -DMACOSX_DEPLOYMENT_TARGET=10.7 -arch x86_64 -fno-stack-check -I/Users/d-fu/vlc-3.0/contrib/x86_64-apple-darwin19/include  -L/Users/d-fu/vlc-3.0/contrib/x86_64-apple-darwin19/lib -Wl,-syslibroot,/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -mmacosx-version-min=10.7 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -DMACOSX_DEPLOYMENT_TARGET=10.7 -arch x86_64 conftest.cpp  >&5
clang: warning: include path for libstdc++ headers not found; pass '-stdlib=libc++' on the command line to use the libc++ standard library instead [-Wstdlibcxx-not-found]
clang: warning: libstdc++ is deprecated; move to libc++ with a minimum deployment target of OS X 10.9 [-Wdeprecated]
ld: library not found for -lstdc++
clang: error: linker command failed with exit code 1 (use -v to see invocation)
configure:4727: $? = 1
configure: program exited with status 1
configure: failed program was:
| /* confdefs.h */
| #define PACKAGE_NAME ""
| #define PACKAGE_TARNAME ""