Revert "build-sys: drop libcap checks, since we don't use caps anymore"

This reverts commit 9ade13604e.

configure.ac

@@ -537,6 +537,25 @@ AC_SUBST(X11_LIBS)
 AC_SUBST(HAVE_X11)
 AM_CONDITIONAL([HAVE_X11], [test "x$HAVE_X11" = x1])
 
+#### Capabilities (optional) ####
+
+CAP_LIBS=''
+
+AC_ARG_WITH(
+        [caps],
+        AS_HELP_STRING([--without-caps],[Omit support for POSIX capabilities.]))
+
+if test "x${with_caps}" != "xno"; then
+    AC_SEARCH_LIBS([cap_init], [cap], [], [
+                    if test "x${with_caps}" = "xyes" ; then
+                        AC_MSG_ERROR([*** POSIX caps libraries not found])
+                    fi])
+    AC_CHECK_HEADERS([sys/capability.h], [], [
+                    if test "x${with_caps}" = "xyes" ; then
+                        AC_MSG_ERROR([*** POSIX caps headers not found])
+                    fi])
+fi
+
 #### Valgrind (optional) ####
 
 AC_CHECK_HEADERS_ONCE([valgrind/memcheck.h])