ceph.spec: use devtoolset-7-gcc-c++ on centos/rhel

for better performance.

Signed-off-by: Kefu Chai <kchai@redhat.com>

ceph.spec.in

@@ -71,6 +71,12 @@ Epoch:		2
 # undefined
 %global _epoch_prefix %{?epoch:%{epoch}:}
 
+%if 0%{?rhel} == 7
+# devtoolset offers newer make and valgrind-devel, but the old ones are good
+# enough.
+%global _devtoolset_prefix devtoolset-7-
+%endif
+
 Summary:	User space components of the Ceph file system
 License:	LGPL-2.1 and CC-BY-SA-1.0 and GPL-2.0 and BSL-1.0 and BSD-3-Clause and MIT
 %if 0%{?suse_version}
@@ -120,7 +126,7 @@ BuildRequires:	gperf
 BuildRequires:  cmake
 BuildRequires:	cryptsetup
 BuildRequires:	fuse-devel
-BuildRequires:	gcc-c++
+BuildRequires:	%{_devtoolset_prefix}gcc-c++
 BuildRequires:	gdbm
 %if 0%{with tcmalloc}
 BuildRequires:	gperftools-devel >= 2.4
@@ -779,6 +785,11 @@ python-rbd, python-rgw or python-cephfs instead.
 %autosetup -p1 -n @TARBALL_BASENAME@
 
 %build
+
+%if 0%{?rhel} == 7
+. /opt/rh/devtoolset-7/enable
+%endif
+
 %if 0%{with cephfs_java}
 # Find jni.h
 for i in /usr/{lib64,lib}/jvm/java/include{,/linux}; do