Conditionalize libdw and libelf linkage to librpmbuild

I clearly remember testing the behavior wrt these and concluding that the
cmake exported interface syntax doesn't require conditionalizing.
Clearly it does though, so dunno what I was supposed to have tested
back then.

Suggested-by: Mark Dascher <mark@papertrail.com>

build/CMakeLists.txt

@@ -23,12 +23,18 @@ target_link_libraries(librpmbuild PUBLIC librpmio librpm)
 target_link_libraries(librpmbuild PRIVATE
 	libmisc
 	PkgConfig::POPT
-	PkgConfig::LIBELF
-	PkgConfig::LIBDW
 	LUA::LUA
 	MAGIC::MAGIC
 )
 
+if (LIBDW_FOUND)
+	target_link_libraries(librpmbuild PRIVATE PkgConfig::LIBDW)
+endif()
+
+if (LIBELF_FOUND)
+	target_link_libraries(librpmbuild PRIVATE PkgConfig::LIBELF)
+endif()
+
 if (Iconv_FOUND)
 	target_link_libraries(librpmbuild PRIVATE Iconv::Iconv)
 endif()