rpm: enforce ".el7" dist tag for CentOS/RHEL 7.

(closes #1337)
diff --git a/rpm/SPECS/nginx-module.spec.in b/rpm/SPECS/nginx-module.spec.in
index 718f6d8..cc0d558 100644
--- a/rpm/SPECS/nginx-module.spec.in
+++ b/rpm/SPECS/nginx-module.spec.in
@@ -17,6 +17,7 @@
 %if 0%{?rhel} == 7
 %define epoch 1
 Epoch: %{epoch}
+%define dist .el7
 %endif
 
 %define main_version %%MAIN_VERSION%%
diff --git a/rpm/SPECS/nginx-plus-module.spec.in b/rpm/SPECS/nginx-plus-module.spec.in
index 55c36f9..b3eba6b 100644
--- a/rpm/SPECS/nginx-plus-module.spec.in
+++ b/rpm/SPECS/nginx-plus-module.spec.in
@@ -14,6 +14,10 @@
 BuildRequires: libopenssl-devel
 %endif
 
+%if 0%{?rhel} == 7
+%define dist .el7
+%endif
+
 %define main_version %%MAIN_VERSION%%
 %define main_release %%MAIN_RELEASE%%%{?dist}.ngx
 
diff --git a/rpm/SPECS/nginx.spec.in b/rpm/SPECS/nginx.spec.in
index c01c2df..9822af0 100644
--- a/rpm/SPECS/nginx.spec.in
+++ b/rpm/SPECS/nginx.spec.in
@@ -34,6 +34,7 @@
 Requires: openssl >= 1.0.1
 BuildRequires: systemd
 BuildRequires: openssl-devel >= 1.0.1
+%define dist .el7
 %endif
 
 %if 0%{?suse_version} == 1315