spkg-configure for cliquer

we do not check the version, as it's quite stable

Upstream's and Sage's cliquer install an executable named `cl`, but it's
not used anywhere in Sage library.
Note that Debian's name for cliquer's CLI is cliquer, not cl.

Author: dimpase

build/pkgs/cliquer/spkg-configure.m4

@@ -0,0 +1,13 @@
+SAGE_SPKG_CONFIGURE([cliquer], [
+    AC_SEARCH_LIBS([clique_unweighted_max_weight], [cliquer], [
+      AC_LANG_PUSH([C])
+      AC_CHECK_HEADER([cliquer/cliquer.h], [
+         AC_PATH_PROG([CL], [cl])
+         AS_IF([test x$CL = x], [
+           AC_MSG_NOTICE([cl (cliquer's CLI) is not found.])
+           AC_MSG_NOTICE([No action, as Sage does not need it. cl might be named cliquer on this system.])
+         ])
+      ], [sage_spkg_install_cliquer=yes])
+      AC_LANG_POP()
+    ], [sage_spkg_install_cliquer=yes])
+])