autotools: don't pass all the libregress cflags to the scanner. Fixes #226
requested to merge creiter/gobject-introspection:autotools-regress-scanner-cflags-cleanup into master
They contain things like -fvisibility=hidden which, in case of building with CFLAGS="-flto -O2" LDFLAGS="-Wl,--as-needed -flto", results in the linker throwing out unused symbols and not linking the dumper against libregress.
This results in errors like: Invalid GType function: 'regress_test_enum_get_type' Failed to find symbol 'regress_test_enum_get_type'
Fix this by only passing the required CFLAGS to the scanner instead.