>%{__mkdir_p} build
Macro forms of system executables should not be used.
https://fedoraproject.org/wiki/Packaging:Guidelines#Macros

>%package devel
>Requires:   %{name} = %{version}-%{release}
This should be an arched dependency - "Requires: %{name}%{?isa}".

(In reply to Artur Iwicki from comment #1)
> >%{__mkdir_p} build
> Macro forms of system executables should not be used.
> https://fedoraproject.org/wiki/Packaging:Guidelines#Macros
>

Fixed
 
> >%package devel
> >Requires:   %{name} = %{version}-%{release}
> This should be an arched dependency - "Requires: %{name}%{?isa}".

Also fixed.

The files can be found now at:
Spec URL: http://marcdeop.fedorapeople.org/libqmatrixclient.spec
SRPM URL: http://marcdeop.fedorapeople.org/libqmatrixclient-0.4.0-2.fc28.src.rpm

Thanks a lot Artur Iwicki!

Here is Koji successful build:
https://koji.fedoraproject.org/koji/taskinfo?taskID=33787382

> Version:  0.4.0
Latest version is 0.4.2.1.

> Summary:  A Qt5 library to write cross-platform clients for Matrix
The leading 'A' is not needed.

> Source0:  https://github.com/QMatrixClient/%{name}/archive/v%{version}.tar.gz
Use a better url to have a properly named tarball:
https://github.com/QMatrixClient/%{name}/archive/v%{version}/%{name}-%{version}.tar.gz

> %package devel
> Requires: %{name}%{?isa}
Requires: %{name}%{?_isa} = %{version}-%{release}

> %build
> mkdir -p build
> pushd build
> %{cmake} ..
> %{__cmake} --build . --target all
> popd
'''
%build
%cmake .
%make_build
'''
is enough.

> %install
> make install/fast DESTDIR=%{buildroot} -C build
%make_install

> %files
> %{license} COPYING
%license, no braces

> %{_libdir}/libQMatrixClient.so.*
Don't glob the soname to avoid un-noticed soname bump.

> %files devel
...snip...
> %{_includedir}/*.h
> %{_includedir}/application-service/definitions/*.h
> %{_includedir}/csapi/*.h
> %{_includedir}/csapi/definitions/*h
> %{_includedir}/csapi/definitions/wellknown/*h
> %{_includedir}/events/*.h
> %{_includedir}/identity/definitions/*.h
> %{_includedir}/jobs/*.h
%{_includedir}/* is enough
But it is not correct to drop all files and dirs directly in %{_includedir}, it should be installed in a subdirectory.

> %changelog
> * Mon Dec 03 2018 Marc Deop marc - 0.4.0-2
> - Do not use mkdir system macro
> - Use arched dependency
> * Sat Nov 24 2018 Marc Deop marc - 0.4.0-1
> - Initial release
The mail needs to be enclosed in <  >.
Add blank lines between changelog entries for legibility.


Also :
- Missing %ldconfig_scriptlets. That is really only necessary for el6 and el7, but expands to nothing on fedora, so it cannot hurt, shall one want EPEL branches.
- CONTRIBUTING.md and possibly others could be shipped as %doc.

Actually, latest version is 0.5.0.2 and the leading 'v' has been stripped from the tag, so the Source0 URL I've provided above needs to be amanded accordingly:
SOURCE0: https://github.com/QMatrixClient/%{name}/archive/%{version}/%{name}-%{version}.tar.gz

It seems the new home for this project is https://github.com/quotient-im/libQuotient and there are newer version tags in the git repo.
Please update.

This was packaged already by someone else under the new name libQuotient: https://koji.fedoraproject.org/koji/packageinfo?packageID=30651