ci: Add Dependabot for GitHub Actions #188

Job	Run time
ubuntu-16.04 BUILD=static CC=gcc-4.7 CXX=g++-4.7 AUTOTOOLS=no	1s
Windows MSVC build (Release, Win64)	5m 24s
Windows MinGW build (shared, x64)	4m 41s
ubuntu-16.04 BUILD=static CC=gcc-4.8 CXX=g++-4.8 AUTOTOOLS=no	1s
Windows MSVC build (Debug, Win64)	4m 53s
Windows MinGW build (static, x64)	6m 14s
ubuntu-16.04 BUILD=static CC=gcc-5 CXX=g++-5 AUTOTOOLS=no	1s
Windows MSVC build (Release, Win32)	6m 31s
Windows MinGW build (shared, x86)	7m 22s
Windows MSVC build (Debug, Win32)	3m 42s
ubuntu-16.04 BUILD=static CC=gcc-6 CXX=g++-6 AUTOTOOLS=no	1s
Windows MinGW build (static, x86)	4m 41s
ubuntu-latest BUILD=static CC=gcc-7 CXX=g++-7 AUTOTOOLS=no	12s
ubuntu-latest BUILD=shared CC=gcc CXX=g++ AUTOTOOLS=yes	25s
ubuntu-latest BUILD=static CC=gcc CXX=g++ AUTOTOOLS=yes	38s
ubuntu-latest BUILD=shared CC=gcc CXX=g++ AUTOTOOLS=no	45s
ubuntu-latest BUILD=static CC=gcc CXX=g++ AUTOTOOLS=no	54s
ubuntu-latest BUILD=shared CC=clang CXX=clang++ AUTOTOOLS=yes	30s
ubuntu-latest BUILD=static CC=clang CXX=clang++ AUTOTOOLS=yes	29s
ubuntu-latest BUILD=shared CC=clang CXX=clang++ AUTOTOOLS=no	1m 18s
ubuntu-latest BUILD=static CC=clang CXX=clang++ AUTOTOOLS=no	1m 16s
macOS-latest BUILD=shared CC=clang CXX=clang++ AUTOTOOLS=yes	54s
macOS-latest BUILD=static CC=clang CXX=clang++ AUTOTOOLS=yes	57s
macOS-latest BUILD=shared CC=clang CXX=clang++ AUTOTOOLS=no	2m 4s
macOS-latest BUILD=static CC=clang CXX=clang++ AUTOTOOLS=no	1m 12s
	55m 6s

Provide feedback