build: don't include libsystemd as shared pkg-config dep when statically linked (#283)

This works whether `BUILD_LIBSYSTEMD` is `ON` or the separate build of
libsystemd picked up by pkg-config is static. The Gentoo Linux package
requires the latter.

This change also handles `BUILD_SHARED_LIBS` being `OFF`. In this case,
the libsystemd dependency does need to be included, even though it is
static.

CMake does not allow you to build shared and static libraries
simultaneously (with a single `add_library` call), but this change
declares the libsystemd dependency as `Requires.private` rather than
omitting it entirely. This is in case someone builds and installs both a
static and shared build and then calls `pkg-config --static sdbus-c++`.
The static build needs to be installed first so that the pkg-config
file from the shared build takes precedence.

CMakeLists.txt

@@ -210,6 +210,11 @@ install(FILES ${CMAKE_CURRENT_BINARY_DIR}/cmake/sdbus-c++-config.cmake
         DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/sdbus-c++
         COMPONENT dev)
 
+if(BUILD_SHARED_LIBS AND (BUILD_LIBSYSTEMD OR Systemd_LINK_LIBRARIES MATCHES "/libsystemd\.a(;|$)"))
+    set(PKGCONFIG_REQS ".private")
+else()
+    set(PKGCONFIG_REQS "")
+endif()
 configure_file(pkgconfig/sdbus-c++.pc.in pkgconfig/sdbus-c++.pc @ONLY)
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/pkgconfig/sdbus-c++.pc
         DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig COMPONENT dev)

pkgconfig/sdbus-c++.pc.in

@@ -5,7 +5,7 @@ includedir=@CMAKE_INSTALL_FULL_INCLUDEDIR@
 
 Name: @PROJECT_NAME@
 Description: C++ library on top of sd-bus, a systemd D-Bus library
-Requires: libsystemd
+Requires@PKGCONFIG_REQS@: libsystemd
 Version: @SDBUSCPP_VERSION@
 Libs: -L${libdir} -l@PROJECT_NAME@
 Cflags: -I${includedir}