build mimic-dev1 with gcc 7

[tchaikov]

backport the GCC-7 related commits to mimic-dev1 in bulk.

[tchaikov]

without this change, among which, 4ddd6b5, the mimic-dev1 build fails like

/home/jenkins-build/build/workspace/ceph-pull-requests/src/rocksdb/db/log_reader.cc: In member function ‘bool rocksdb::log::Reader::ReadRecord(rocksdb::Slice*, std::__cxx11::string*, rocksdb::WALRecoveryMode)’:
/home/jenkins-build/build/workspace/ceph-pull-requests/src/rocksdb/db/log_reader.cc:150:9: error: this statement may fall through [-Werror=implicit-fallthrough=]
         if (wal_recovery_mode == WALRecoveryMode::kAbsoluteConsistency) {
         ^~
/home/jenkins-build/build/workspace/ceph-pull-requests/src/rocksdb/db/log_reader.cc:156:7: note: here
       case kEof:
       ^~~~
/home/jenkins-build/build/workspace/ceph-pull-requests/src/rocksdb/db/log_reader.cc:170:9: error: this statement may fall through [-Werror=implicit-fallthrough=]
         if (wal_recovery_mode != WALRecoveryMode::kSkipAnyCorruptedRecords) {
         ^~
/home/jenkins-build/build/workspace/ceph-pull-requests/src/rocksdb/db/log_reader.cc:186:7: note: here
       case kBadRecord:
       ^~~~
cc1plus: all warnings being treated as errors

see https://jenkins.ceph.com/job/ceph-pull-requests/38248/consoleFull#-20636943662a811ea2-3e7b-466b-84b4-d13df7e35809 (#19544)

[liewegas]

Most recent build seems okay? https://shaman.ceph.com/repos/ceph/mimic-dev1/255453442fcc28e79f60bafafee06a4b0f79cffc/

[tchaikov]

@liewegas these are two different jenkins jobs for building ceph,

• the "make check" job: it builds pull requests on ceph/ceph, and it uses run-make-check.sh to do this job, install-deps.sh is used by this script to ready the building env. if install-deps.sh fails to do so, the build job fails. currently, the xenial building slaves does not necessarily have gcc-7 installed or have its /usr/bin/gcc pointed to /usr/bin/gcc-7, so we need to ensure this.
• the "package-and-send-them-to-chacra" job: it builds ceph/ceph-ci branches. the link you posted in the above comment falls into this category.