gnu: lua-5.1: install pkg-config file.

* gnu/packages/lua.scm (lua-5.1): Modified.
* gnu/packages/patches/lua51-pkgconfig.patch: New file.
---
 gnu/local.mk                               |   1 +
 gnu/packages/lua.scm                       |   3 +-
 gnu/packages/patches/lua51-pkgconfig.patch | 101 +++++++++++++++++++++++++++++
 3 files changed, 104 insertions(+), 1 deletion(-)
 create mode 100644 gnu/packages/patches/lua51-pkgconfig.patch