Always ignore ldconfig failures

PR cossacklabs#346 [1] has improved ldconfig handling a bit by failing the build
only when ldconfig is invoked as root. However, this causes issues when
building under fakeroot (which sets EUID but does not give permissions).
Furthemore, GNU Makefile conventions [2] expect ldconfig failures to be
simply ignored.

While we're here, make ldconfig binary configurable by providing
LDCONFIG variable. This may be used by some packaging software.
However, keep invoking ldconfig only on Linux as that's where it's
required to be run after installation.

[1]: cossacklabs#346
[2]: https://www.gnu.org/prep/standards/html_node/Utilities-in-Makefiles.html#Utilities-in-Makefiles

Makefile

@@ -17,6 +17,7 @@
 #CC = clang
 CLANG_FORMAT ?= clang-format
 CLANG_TIDY   ?= clang-tidy
+LDCONFIG     ?= ldconfig
 SHELL = /bin/bash
 SRC_PATH = src
 ifneq ($(BUILD_PATH),)
@@ -451,7 +452,7 @@ install_pkgconfig: err all make_install_dirs
 
 install: install_soter_headers install_themis_headers install_static_libs install_shared_libs install_pkgconfig
 ifdef IS_LINUX
-	@ldconfig || (status=$$?; if [ $$(id -u) = "0" ]; then exit $$status; else exit 0; fi)
+	@$(LDCONFIG) || true
 endif
 
 get_version: