Trac #29430: new spkg-configure.m4 for flintqs.

The flintqs package installs the QuadraticSieve executable that is
used (and assumed to be in PATH) by the qsieve() command in SageMath.
This commit adds an spkg-configure.m4 that checks for the existence
of QuadraticSieve in the user's PATH. We don't do any version checks
because there's only one version of flintqs, and its predecessor,

  http://www.friedspace.com/QS/

was last updated in 2007.

build/pkgs/flintqs/spkg-configure.m4

@@ -0,0 +1,13 @@
+SAGE_SPKG_CONFIGURE([flintqs], [
+  SAGE_SPKG_DEPCHECK([gmp], [
+    # FlintQS only builds against GMP, not MPIR. If we're using the
+    # system GMP, then we check for the QuadraticSieve program, which
+    # is the only interface to FlintQS that sagelib uses.
+    AC_CHECK_PROG(HAVE_QUADRATICSIEVE, QuadraticSieve, yes, no)
+    AS_IF([test "x$HAVE_QUADRATICSIEVE" = "xno"],
+          [sage_spkg_install_flintqs=yes])
+  ],
+  [ # If we're using sage's GMP, we have to use its FlintQS, too.
+    sage_spkg_install_flintqs=yes
+  ])
+])