Makefile and docs: add GNU Make Standard Library
mxe configury: build native pkgconf and centralise various conf files

Since we need pkgconf for Cflags.private support, it seems like a
logical place for these types of config files instead of being
spread over Makefile, binutils, and gcc.
Makefile: refactor lookup of pkg build rules and deps

Introduces a LOOKUP_PKG_RULE function that matches generic rules
from targets specified along the lines of:

triplet.lib-type.debug.treading.exceptions.my.experimental.version

Current build rules will be matched according to the current logic
(pkg-target --> pkg) and future variants can use "targets" as both
functionality determinants and a directory separator for multiple
side-by-side variants.
packages gcc sdl_rwhttp: use new-style target-specific deps
Merge pull request #319 from tonytheodore/mxe-configury

mxe configury: build native pkgconf and centralise various conf files
Merge pull request #320 from tonytheodore/lookup-pkg-rule

Makefile: refactor lookup of pkg build rules and deps
Makefile docs and pkgconf: add support for shared|static variants
packages gmp mpfr mpc: enable shared builds and runtime tests
package qt4 and dependencies: enable shared builds
Makefile and pkgconf: move external tools separate directory
package hdf5: disable shared build and workaround for generated sources
package poppler: fix PKG_CONFIG_PATH_*
Updated FFTW3 tp support shared building
Updated FLAC to support shared building
Updated libsamplerate to support shared building
Updated libxml2 to support shared building
Updated libsdnfile to support shared building
Updated Cairo for shared build
Updated Fribidi for shared build
Updated Glib for shared build