build: enforce in-source build for rpmbuild

The out-of-source build is the default mode since Fedora 33 (see [1]).
It is easier to disable it rather than fix `make` / `make check` / `make
install` invocations in a distribution agnostic way: the new macros
introduced by Fedora are not available in CentOS 7.

[1]: https://fedoraproject.org/wiki/Changes/CMake_to_do_out-of-source_builds

Part of #34

Author: Totktonada

rpm/tarantool-smtp.spec

@@ -21,7 +21,7 @@ This package provides SMTP client module for Tarantool.
 %setup -q -n %{name}-%{version}
 
 %build
-%cmake . -DCMAKE_BUILD_TYPE=RelWithDebInfo
+%cmake -B . -DCMAKE_BUILD_TYPE=RelWithDebInfo
 make %{?_smp_mflags}
 
 %check