Make editline and lowdown optional in configure.ac

configure.ac

@@ -254,14 +254,29 @@ PKG_CHECK_MODULES([LIBCURL], [libcurl], [CXXFLAGS="$LIBCURL_CFLAGS $CXXFLAGS"])
 # Look for editline, a required dependency.
 # The the libeditline.pc file was added only in libeditline >= 1.15.2,
 # see https://github.com/troglobit/editline/commit/0a8f2ef4203c3a4a4726b9dd1336869cd0da8607,
-# but e.g. Ubuntu 16.04 has an older version, so we fall back to searching for
-# editline.h when the pkg-config approach fails.
-PKG_CHECK_MODULES([EDITLINE], [libeditline], [CXXFLAGS="$EDITLINE_CFLAGS $CXXFLAGS"], [
-  AC_CHECK_HEADERS([editline.h], [true],
-    [AC_MSG_ERROR([Nix requires libeditline; it was found neither via pkg-config nor its normal header.])])
-  AC_SEARCH_LIBS([readline read_history], [editline], [],
-    [AC_MSG_ERROR([Nix requires libeditline; it was not found via pkg-config, but via its header, but required functions do not work. Maybe it is too old? >= 1.14 is required.])])
-])
+# Older versions are no longer supported.
+AC_ARG_ENABLE(
+  editline,
+  AS_HELP_STRING([--enable-editline],[Enable nice line editting in the nix repl [default=auto]]),
+  [],
+  [enable_editline=auto])
+AS_CASE(["$enable_editline"],
+  [yes | auto],
+  [
+    PKG_CHECK_MODULES(
+      [EDITLINE],
+      [libeditline],
+      [CXXFLAGS="$EDITLINE_CFLAGS $CXXFLAGS"],
+      [
+	    AS_IF([test "x$enable_editline" == "xyes"],[
+          AC_MSG_ERROR([You passed --enable-editline=yes, requiring libeditline, but it was not found.])
+        ])
+      ])
+  ],
+  [no],
+  [],
+  [*],
+  [AC_MSG_ERROR([bad value $enable_editline for --enable-editline])])
 
 # Look for libsodium.
 PKG_CHECK_MODULES([SODIUM], [libsodium], [CXXFLAGS="$SODIUM_CFLAGS $CXXFLAGS"])
@@ -369,7 +384,28 @@ PKG_CHECK_MODULES([NLOHMANN_JSON], [nlohmann_json >= 3.9])
 
 
 # Look for lowdown library.
-PKG_CHECK_MODULES([LOWDOWN], [lowdown >= 0.9.0], [CXXFLAGS="$LOWDOWN_CFLAGS $CXXFLAGS"])
+AC_ARG_ENABLE(
+  lowdown,
+  AS_HELP_STRING([--enable-lowdown],[Enable editline for in-process docs support]]),
+  [],
+  [enable_lowdown=auto])
+AS_CASE(["$enable_lowdown"],
+  [yes | auto],
+  [
+    PKG_CHECK_MODULES(
+      [LOWDOWN],
+      [liblowdown >= 0.9.0],
+      [CXXFLAGS="$LOWDOWN_CFLAGS $CXXFLAGS"],
+      [
+	    AS_IF([test "x$enable_lowdown" == "xyes"],[
+          AC_MSG_ERROR([You passed --enable-lowdown=yes, requiring liblowdown, but it was not found.])
+        ])
+      ])
+  ],
+  [no],
+  [],
+  [*],
+  [AC_MSG_ERROR([bad value $enable_lowdown for --enable-lowdown])])
 
 
 # Look for libgit2.