Set library and header installation directories from configuration

Set library and header installation directories from configuration

Currently the installation directories for libraries and headers are
resolved within the install commands. For instance, the libraries will
be installed to $(INSTALL_DIR)/$(LIB_DIR). This limits the flexibility
of the installation, such as when the libraries should be installed to a
subdirectory like /usr/lib/tls.

This adds the make variables $(INSTALL_LIB_DIR) and $(INSTALL_INC_DIR)
to define the locations that the libraries and headers are installed.
For the static configs, this resolves exactly as before to
$(INSTALL_DIR)/include and $(INSTALL_DIR)/$(LIB_DIR). For autoconf, they
are derived directly from the --libdir and --includedir settings.