build: Add a new --with-devlibdir configure option

This will make it possible to use a different directory for the C libdpkg
files to the dselect methods and perl plugins directory.

configure.ac

@@ -33,6 +33,8 @@ DPKG_BUILD_PROG([start-stop-daemon])
 DPKG_BUILD_PROG([update-alternatives])
 
 # Allow alternate directories
+DPKG_WITH_DIR([devlibdir], [${libdir}],
+              [dpkg development library directory [LIBDIR]])
 DPKG_WITH_DIR([pkgconfdir], [${sysconfdir}/${PACKAGE_NAME}],
               [dpkg configuration directory [SYSCONFDIR/dpkg]])
 DPKG_WITH_DIR([admindir], [${localstatedir}/lib/${PACKAGE_NAME}],

debian/changelog

@@ -31,6 +31,8 @@ dpkg (1.18.3) UNRELEASED; urgency=low
     - Get the reference build flags from dpkg-buildflags.pl, instead of
       hardcoding them, which might not match depending on the architecture.
       Closes: #794694
+  * Build system:
+    - Add a new --with-devlibdir configure option for the C libdpkg library.
   * Packaging:
     - Remove unneeded --sourcedir options from dh_install calls.
     - Do not change libdir to a multi-arch directory for the entire dpkg.

lib/dpkg/Makefile.am

@@ -18,10 +18,10 @@ EXTRA_DIST = \
 	libdpkg.map \
 	libdpkg.pc.in
 
-pkgconfigdir = $(libdir)/pkgconfig
+pkgconfigdir = $(devlibdir)/pkgconfig
 pkgconfig_DATA = libdpkg.pc
 
-lib_LTLIBRARIES = libdpkg.la
+devlib_LTLIBRARIES = libdpkg.la
 
 libdpkg_la_LDFLAGS =
 if HAVE_LINKER_VERSION_SCRIPT

lib/dpkg/libdpkg.pc.in

@@ -1,6 +1,6 @@
 prefix=@prefix@
 exec_prefix=@exec_prefix@
-libdir=@libdir@
+libdir=@devlibdir@
 includedir=@includedir@
 
 Name: libdpkg