fromYAML: let configure fail if rapidyaml cannot be found

configure.ac

@@ -200,31 +200,25 @@ PKG_CHECK_MODULES([EDITLINE], [libeditline], [CXXFLAGS="$EDITLINE_CFLAGS $CXXFLA
     [AC_MSG_ERROR([Nix requires libeditline; it was not found via pkg-config, but via its header, but required functions do not work. Maybe it is too old? >= 1.14 is required.])])
 ])
 
-# Look for rapidyaml, an optional dependency.
+# Look for rapidyaml.
 have_ryml=
-AC_ARG_ENABLE([ryml], AS_HELP_STRING([--disable-ryml], [Do not enable rapidyaml and disable builtins.fromYAML]))
+AC_ARG_ENABLE([ryml], AS_HELP_STRING([--disable-ryml], [Do not enable rapidyaml and disable builtins.fromYAML], [have_ryml=1]))
 AC_ARG_VAR([RYML_CPPFLAGS], [C/C++ preprocessor flags for RAPIDYAML])
 AC_ARG_VAR([RYML_LDFLAGS], [linker flags for RAPIDYAML])
-if test "x$enable_ryml" != "xno"; then
+if test "x$have_ryml" != "x"; then
     AC_LANG_PUSH([C++])
-    saveCXXFLAGS="$CXXFLAGS"
-    saveLDFLAGS="$LDFLAGS"
-    saveLIBS="$LIBS"
     # append RYML_CPPFLAGS to CXXFLAGS because CPPFLAGS are not passed to the C++ compiler
     CXXFLAGS="$RYML_CPPFLAGS $CXXFLAGS"
     LDFLAGS="$RYML_LDFLAGS $RYML_LDFLAGS"
     LIBS="-lryml $LIBS"
-    AC_CHECK_HEADERS([ryml.hpp], [have_ryml=1], [])
+    AC_CHECK_HEADERS([ryml.hpp], [true],
+      [AC_MSG_ERROR([Header of libryml is not found.])])
     AC_LINK_IFELSE([AC_LANG_PROGRAM([[
         #include <ryml.hpp>
       ]], [[ryml::Tree tree;]])],
       [AC_DEFINE([HAVE_RYML], [1], [Use rapidyaml])
        AC_SUBST(RYML_LIBS, [-lryml])],
-      [have_ryml=
-       CXXFLAGS="$saveCXXFLAGS"
-       LDFLAGS="$saveLDFLAGS"
-       LIBS="$saveLIBS"
-       AC_MSG_RESULT([rapidyaml is not available])])
+      [AC_MSG_ERROR([libryml is not found.])])
     AC_LANG_POP([C++])
 fi
 AC_SUBST(HAVE_RYML, [$have_ryml])