Although I can't explain the appearance of the -Wno-framework-include-private-from-public flag in it, I'm not sure that the failure of the gcc -V test matters; that test is supposed to fail anyway.
Still, it might be useful to check your environment variables (run env) and make sure you don't have any variables set that would influence the build, such as CC, CXX, CFLAGS, CPPFLAGS, CXXFLAGS, LDFLAGS, DYLD_LIBRARY_PATH. It's also a good idea to make sure your PATH doesn't include non-Apple components. For example try export PATH=/usr/bin:/bin:/usr/sbin:/sbin before configuring MacPorts. Also make sure you don't have anything in /usr/local or /Library/Frameworks.
Later in the log, I see that the compile checks seem to fail because of errors in the SDK:
In file included from conftest.c:14:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/string.h:63:
In file included from /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/Availability.h:246:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/AvailabilityInternal.h:147:2: error: #endif without #if
#endif /* __ENABLE_LEGACY_IPHONE_AVAILABILITY */
^
Of course I'm sure your Xcode SDK files are fine. Is it possible that you have an old version of the command line tools? Make sure you've installed the Xcode 11.1-compatible version of the command line tools for Catalina.
Others are successfully building MacPorts 2.6.1 on Catalina so something must be different about your computer.
