This bug appears to have been reported against 'rawhide' during the Fedora 19 development cycle.
Changing version to '19'.

(As we did not run this process for some time, it could affect also pre-Fedora 19 development
cycle bugs. We are very sorry. It will help us with cleanup during Fedora 19 End Of Life. Thank you.)

More information and reason for this action is here:
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This message is a notice that Fedora 19 is now at end of life. Fedora 
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Thank you for reporting this issue and we are sorry that we were not 
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of Fedora, you are encouraged  change the 'version' to a later Fedora 
version prior this bug is closed as described in the policy above.

Although we aim to fix as many bugs as possible during every release's 
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Or pkgconfig might provide such information at least ...

commit e914597c18f918668cf0381f59d7c8117b17688d adds %_systemtap_tapsetdir

(In reply to Frank Ch. Eigler from comment #4)
> commit e914597c18f918668cf0381f59d7c8117b17688d 

No workie with rhel7 and older, where %{_rpmmacrodir} evals to empty string. `make rpm` then ends up with:

=======
RPM build errors:
    File must begin with "/": %{_rpmmacrodir}/macros.systemtap
make: *** [rpm] Error 1
=======

Tested following possible fix on rhel-6+ and Fedora-28:

=======
  diff --git a/systemtap.spec b/systemtap.spec
index 4392b1b..61c09bb 100644
--- a/systemtap.spec
+++ b/systemtap.spec
@@ -73,6 +73,12 @@
     %define dracutbindir %{_bindir}
 %endif
 
+%if 0%{?rhel} == 6
+    %{!?_rpmmacrodir: %define _rpmmacrodir /etc/rpm/}
+%else
+    %{!?_rpmmacrodir: %define _rpmmacrodir %{_rpmconfigdir}/macros.d}
+%endif
+
 # To avoid testsuite/*/*.stp has shebang which doesn't start with '/'
 %undefine __brp_mangle_shebangs
=======

This by default
- (always allows overriding _rpmmacrodir with a custom value)
- on rhel6 does %define _rpmmacrodir /etc/rpm/
- on rhel7 does %define _rpmmacrodir %{_rpmconfigdir}/macros.d (evals to /usr/lib/rpm/macros.d)
- on fedora does nothing

With this, %{_systemtap_tapsetdir} gets defined on rhel6+ and fedora as expected.

(In reply to Martin Cermak from comment #5)
> =======
>   diff --git a/systemtap.spec b/systemtap.spec
> index 4392b1b..61c09bb 100644
> --- a/systemtap.spec
> +++ b/systemtap.spec
> @@ -73,6 +73,12 @@
>      %define dracutbindir %{_bindir}
>  %endif
>  
> +%if 0%{?rhel} == 6
> +    %{!?_rpmmacrodir: %define _rpmmacrodir /etc/rpm/}
> +%else
> +    %{!?_rpmmacrodir: %define _rpmmacrodir %{_rpmconfigdir}/macros.d}
> +%endif
> +
>  # To avoid testsuite/*/*.stp has shebang which doesn't start with '/'
>  %undefine __brp_mangle_shebangs
> =======

The RHEL condition is useless IMO. Just test the existence of macros and act upon it.

Problem is that on rhel-6, %{_rpmconfigdir}/macros.d evals to /usr/lib/rpm/macros.d, but rpm doesn't actually use it.  Instead, on rhel6, rpm loads it's macros from /etc/rpm/.  So rhel-6 is sort of a special case, I'd say.

(In reply to Martin Cermak from comment #7)
Ah, I did not know that. In that case it seems reasonable ...

Fixed in commit 9297d41f049795323066314d70b82c98a750cb8c .