external: add some READMEs

Change-Id: If4ba9ff89d3fa1e092bc20abb72a9a0dbef5396a

external/firebird/README

+Firebird is an embeddable SQL RDBMS implemented in C++
+
+http://firebirdsql.org/

external/glew/README

+GLEW is the OpenGL Extension Wrangler
+
+GLEW provides efficient run-time mechanisms for determining which OpenGL
+extensions are supported on the target platform.
+
+http://glew.sourceforge.net/

external/glyphy/README

+GLyphy is a text renderer that uses OpenGL
+
+https://github.com/behdad/glyphy

external/openldap/README

+OpenLDAP provides an LDAP client library
+
+http://www.openldap.org/

external/ucpp/README

+ucpp is a C99 preprocessor
+
+Used to be hosted at https://code.google.com/p/ucpp/ - not sure where it's gone