libepoxy 1.5.1 release.

• Do no add pkg-config dependencies on gl on systems that do not use pkg-config, like macOS and Windows
• Generalise checks for dlvsym
• Add an option for disabling building the test suite
• Typo fixes in the comments and documentation
• Simplify the Meson configuration logic for EGL and GLX
• Use assert when no context is found
• Remove a test superceded by GLVND
• Avoid Meson warnings when testing for linker arguments