Travis: add link-time optimisation to detect function signature mismatches
For #27772 (moved), nickm used:
./configure CC=clang CFLAGS='-O2 -flto' LDFLAGS='-flto' RANLIB=/bin/true AR=llvm-ar NM=llvm-nm
For #27772 (moved), nickm used:
./configure CC=clang CFLAGS='-O2 -flto' LDFLAGS='-flto' RANLIB=/bin/true AR=llvm-ar NM=llvm-nm