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.
diff --git a/configure.ac b/configure.ac
index f15d9ff..961a60c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -994,6 +994,7 @@
 echo "        prefix:          $prefix"
 echo "        exec_prefix:     $exec_prefix"
 echo "        libdir:          $libdir"
+echo "        includedir:      $includedir"
 
 dnl Driver info
 echo ""