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.
16 files changed