macros: add new distribution-specific macros for package configurations

This commit is based on

https://src.fedoraproject.org/rpms/fedora-release/pull-request/223

,

customized for CentOS.

Define macros for commonly-used strings/urls to distinguish
distributions. Spec files have lots of `%{?fedora:}` and `%{?rhel}`
conditionals which can be defined in one place.

Amazon Linux is a new downstream distribution of Fedora that will add
its own definitions as well, so consolidate all of these in a new macros
file.

As examples, this is how clang.spec and gdb.spec will look after this
change:

clang.spec:

```
@@ -358,7 +360,7 @@ CFLAGS="$CFLAGS -Wno-address -Wno-nonnull -Wno-maybe-uninitialized"
        \
        -DCLANG_BUILD_EXAMPLES:BOOL=OFF \
        -DBUILD_SHARED_LIBS=OFF \
-       -DCLANG_REPOSITORY_STRING="%{?fedora:Fedora}%{?rhel:Red Hat} %{version}-%{release}" \
+       -DCLANG_REPOSITORY_STRING="%{?dist_vendor} %{version}-%{release}" \
 %ifarch %{arm}
        -DCLANG_DEFAULT_LINKER=lld \
 %endif
```

gdb.spec:

```
-# Change the version that gets printed at GDB startup, so it is RH specific.
+# Change the version that gets printed at GDB startup, so it is distro-specific.
 cat > gdb/version.in << _FOO
-%if 0%{!?rhel:1}
-Fedora %{version}-%{release}
-%else # !0%{!?rhel:1}
-Red Hat Enterprise Linux %{version}-%{release}
-%endif # !0%{!?rhel:1}
+%{?dist_name} %{version}-%{release}
 _FOO

 # Remove the info and other generated files added by the FSF release
```

Signed-off-by: Amit Shah <amitshah@fedoraproject.org>