normalize variable names once and for all in rock_add_plain_dependency
be consistent in our handling of pkg-config and cmake dependencies

in CMake dependencies, CFLAGS only contain definitions and no include
path, and _INCLUDE_DIR or _INCLUDE_DIRS the include paths. pkg-config
provides _CFLAGS as definitions + include paths and _INCLUDE_DIRS for
the pure includes, as well as CFLAGS_OTHER for the flags without
includes.

Copy CFLAGS into CFLAGS_OTHER in rock_add_plain_dependency, and handle
it properly in pkg-config generation
move setting the temporary variables for pkg-config file generation to rock_prepare_pkgconfig

These are temporary, and must therefore be generated just before the
generation itself.
Merge pull request #2 from rock-core/fix_propagation_of_cmake_dependencies_to_the_generated_pkgconfig_files

Fix propagation of cmake dependencies to the generated pkgconfig files