Add missing file (#17426)

Co-authored-by: Ivan Razumov <ivan.razumov@cern.ch>

var/spack/repos/builtin/packages/rivet/gsl.m4

+# AC_SEARCH_GSL(actionIfFound, actionIfNotFound)
+AC_DEFUN([AC_SEARCH_GSL],[
+  AC_ARG_WITH([gsl], AC_HELP_STRING(--with-gsl, [path to GSL library and header files]))
+
+  ## Use a specified --with-gsl arg to set basic paths, if provided
+  GSLCONFIG_PATH=$PATH
+  if test -e "$with_gsl"; then
+    GSLCONFIG_PATH="$with_gsl/bin:$GSLCONFIG_PATH"
+    GSLPATH="$with_gsl"
+    GSLINCPATH="$GSLPATH/include"
+    GSLLIBPATH="$GSLPATH/lib"
+    GSL_CPPFLAGS="-I$GSLINCPATH"
+    GSL_CXXFLAGS=""
+    GSL_LDFLAGS="-L$GSLPATH/lib -lgsl -lgslcblas -lm"
+  fi
+
+  ## Try to do better, using the gsl-config script
+  AC_PATH_PROG(GSLCONFIG, gsl-config, [], [$GSLCONFIG_PATH])
+  if test -x "$GSLCONFIG"; then
+    AC_MSG_NOTICE(Using $GSLCONFIG to find GSL flags)
+    GSLPATH=`$GSLCONFIG --prefix`
+    GSLINCPATH="$GSLPATH/include"
+    GSLLIBPATH="$GSLPATH/lib"
+    GSL_CPPFLAGS=`$GSLCONFIG --cflags`
+    GSL_CXXFLAGS=`$GSLCONFIG --cflags`
+    GSL_LDFLAGS=`$GSLCONFIG --libs`
+  fi
+
+  ## If it's worked, propagate the conditionals and execute success arg
+  if test -e "$GSLPATH"; then
+    AM_CONDITIONAL([WITH_GSL], true)
+    AM_CONDITIONAL([WITH_GSLLIB], true)
+    AM_CONDITIONAL([WITH_GSLINC], true)
+    AM_CONDITIONAL([WITHOUT_GSL], false)
+    AM_CONDITIONAL([WITHOUT_GSLLIB], false)
+    AM_CONDITIONAL([WITHOUT_GSLINC], false)
+    $1
+  else
+    ## Otherwise execute the fail arg
+    AM_CONDITIONAL([WITH_GSL], false)
+    AM_CONDITIONAL([WITH_GSLLIB], false)
+    AM_CONDITIONAL([WITH_GSLINC], false)
+    AM_CONDITIONAL([WITHOUT_GSL], true)
+    AM_CONDITIONAL([WITHOUT_GSLLIB], true)
+    AM_CONDITIONAL([WITHOUT_GSLINC], true)
+    $2
+  fi
+
+  ## Propagate path and flag variables
+  AC_SUBST([GSLPATH])
+  AC_SUBST([GSLINCPATH])
+  AC_SUBST([GSLLIBPATH])
+  AC_SUBST([GSL_CPPFLAGS])
+  AC_SUBST([GSL_CXXFLAGS])
+  AC_SUBST([GSL_LDFLAGS])
+  AC_MSG_NOTICE([GSL include path is $GSLINCPATH])
+  AC_MSG_NOTICE([GSL CPPFLAGS is $GSL_CPPFLAGS])
+  AC_MSG_NOTICE([GSL CXXFLAGS is $GSL_CXXFLAGS])
+  AC_MSG_NOTICE([GSL LDFLAGS is $GSL_LDFLAGS])
+])