Follow GNU conventions for installation: ThemisPP

First of all, there are quite a few ThemisPP-specific rules in the main
Makefile. Move them all into a separate one, just for ThemisPP.

After that, update ThemisPP installation targets to follow the same
conventions as for Soter and Themis.

Makefile

@@ -294,6 +294,7 @@ include src/soter/soter.mk
 include src/themis/themis.mk
 ifndef CARGO
 include src/wrappers/themis/jsthemis/jsthemis.mk
+include src/wrappers/themis/themispp/themispp.mk
 include jni/themis_jni.mk
 endif
 endif
@@ -350,11 +351,6 @@ $(OBJ_PATH)/%.fmt_check: %
 	@echo -n "check $< "
 	@$(BUILD_CMD_)
 
-THEMISPP_HEADERS = $(wildcard $(SRC_PATH)/wrappers/themis/themispp/*.hpp)
-
-FMT_FIXUP += $(patsubst %,$(OBJ_PATH)/%.fmt_fixup, $(THEMISPP_HEADERS))
-FMT_CHECK += $(patsubst %,$(OBJ_PATH)/%.fmt_check, $(THEMISPP_HEADERS))
-
 #$(AUD_PATH)/%: CMD = $(CC) $(CFLAGS) -E -dI -dD $< -o $@
 $(AUD_PATH)/%: CMD = ./scripts/pp.sh  $< $@
 
@@ -521,19 +517,6 @@ endif
 	@echo -n "pythemis install "
 	@$(BUILD_CMD_)
 
-
-themispp_install: CMD = install $(SRC_PATH)/wrappers/themis/themispp/*.hpp $(PREFIX)/include/themispp
-
-themispp_install:
-	@mkdir -p $(PREFIX)/include/themispp
-	@$(BUILD_CMD)
-
-themispp_uninstall: CMD = rm -rf $(PREFIX)/include/themispp
-
-themispp_uninstall:
-	@echo -n "themispp uninstall "
-	@$(BUILD_CMD_)
-
 soter_collect_headers:
 	@mkdir -p $(BIN_PATH)/include/soter
 	@cd src/soter && find . -name \*.h -exec cp --parents {} ../../$(BIN_PATH)/include/soter/ \; && cd - > /dev/null

src/wrappers/themis/themispp/themispp.mk

@@ -0,0 +1,31 @@
+#
+# Copyright (c) 2019 Cossack Labs Limited
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+THEMISPP_HEADERS = $(wildcard $(SRC_PATH)/wrappers/themis/themispp/*.hpp)
+
+FMT_FIXUP += $(patsubst %,$(OBJ_PATH)/%.fmt_fixup, $(THEMISPP_HEADERS))
+FMT_CHECK += $(patsubst %,$(OBJ_PATH)/%.fmt_check, $(THEMISPP_HEADERS))
+
+themispp_install:
+	@echo -n "install ThemisPP "
+	@mkdir -p $(DESTDIR)/$(includedir)/themispp
+	@$(INSTALL_DATA) $(SRC_PATH)/wrappers/themis/themispp/*.hpp $(DESTDIR)/$(includedir)/themispp
+	@$(PRINT_OK_)
+
+themispp_uninstall:
+	@echo -n "uninstall ThemisPP "
+	@rm -rf $(DESTDIR)/$(includedir)/themispp
+	@$(PRINT_OK_)