use AC_CHECK_LIB() calls, as they can be nested

Nesting AC_SEARCH_LIBS with the same 1st arg is apparently
a no-no.

Also, add the new generated file to the lists of to be cleaned
and ignored files.

.gitignore

@@ -95,4 +95,5 @@ gitlab-build-docker.log
 /src/build
 /src/Makefile
 /src/bin/sage-env-config
+/src/sage/env.py
 

Makefile

@@ -91,7 +91,7 @@ distclean: build-clean
 	$(MAKE) misc-clean
 	@echo "Deleting all remaining output from build system ..."
 	rm -rf local
-	rm -f src/bin/sage-env-config
+	rm -f src/bin/sage-env-config src/sage/env.py
 
 # Delete all auto-generated files which are distributed as part of the
 # source tarball

build/pkgs/arb/spkg-configure.m4

@@ -8,12 +8,10 @@ SAGE_SPKG_CONFIGURE([arb], [
     else
         AC_CHECK_HEADER(arb.h, [
         dnl below function added in version 2.16 of arb
-            AC_SEARCH_LIBS([acb_mat_eig_simple], [arb], [
-	      ],[dnl in Debian the name of dylib is different.
-              AC_SEARCH_LIBS([acb_mat_eig_simple], [flint-arb], [
-                  SAGE_ARB_LIBRARY="flint-arb"
-	        ], [sage_spkg_install_arb=yes])
-	    ])
+            AC_CHECK_LIB([arb], [acb_mat_eig_simple], [],
+	      [dnl in Debian the name of dylib is different.
+               AC_CHECK_LIB([flint-arb], [acb_mat_eig_simple],
+                [SAGE_ARB_LIBRARY="flint-arb"], [sage_spkg_install_arb=yes])])
         ], [sage_spkg_install_arb=yes])
     fi
     AC_SUBST(ARB_LIBRARY,[$SAGE_ARB_LIBRARY])