Make Missing Modules.symvers Fatal

Detect early on in configure if the Modules.symvers file is missing.
Without this file there will be build failures later and it's best
to catch this early and provide a useful error.  In this case the
most likely problem is the kernel-devel packages are not installed.
It may also be possible that they are using an unbuilt custom kernel
in which case they must build the kernel first.

Closes #127

config/kernel.m4

@@ -61,6 +61,14 @@ AC_DEFUN([ZFS_AC_MODULE_SYMVERS], [
 		else
 			LINUX_SYMBOLS=Module.symvers
 		fi
+
+		if ! test -f "$LINUX_OBJ/$LINUX_SYMBOLS"; then
+			AC_MSG_ERROR([
+	*** Please make sure the kernel devel package for your distribution
+	*** is installed.  If your building with a custom kernel make sure the
+	*** kernel is configured, built, and the '--with-linux=PATH' configure
+	*** option refers to the location of the kernel source.])
+		fi
 	else
 		LINUX_SYMBOLS=NONE
 	fi

configure

@@ -11964,6 +11964,20 @@ $as_echo_n "checking kernel file name for module symbols... " >&6; }
 		else
 			LINUX_SYMBOLS=Module.symvers
 		fi
+
+		if ! test -f "$LINUX_OBJ/$LINUX_SYMBOLS"; then
+			{ { $as_echo "$as_me:$LINENO: error:
+	*** Please make sure the kernel devel package for your distribution
+	*** is installed.  If your building with a custom kernel make sure the
+	*** kernel is configured, built, and the '--with-linux=PATH' configure
+	*** option refers to the location of the kernel source." >&5
+$as_echo "$as_me: error:
+	*** Please make sure the kernel devel package for your distribution
+	*** is installed.  If your building with a custom kernel make sure the
+	*** kernel is configured, built, and the '--with-linux=PATH' configure
+	*** option refers to the location of the kernel source." >&2;}
+   { (exit 1); exit 1; }; }
+		fi
 	else
 		LINUX_SYMBOLS=NONE
 	fi
@@ -15955,6 +15969,20 @@ $as_echo_n "checking kernel file name for module symbols... " >&6; }
 		else
 			LINUX_SYMBOLS=Module.symvers
 		fi
+
+		if ! test -f "$LINUX_OBJ/$LINUX_SYMBOLS"; then
+			{ { $as_echo "$as_me:$LINENO: error:
+	*** Please make sure the kernel devel package for your distribution
+	*** is installed.  If your building with a custom kernel make sure the
+	*** kernel is configured, built, and the '--with-linux=PATH' configure
+	*** option refers to the location of the kernel source." >&5
+$as_echo "$as_me: error:
+	*** Please make sure the kernel devel package for your distribution
+	*** is installed.  If your building with a custom kernel make sure the
+	*** kernel is configured, built, and the '--with-linux=PATH' configure
+	*** option refers to the location of the kernel source." >&2;}
+   { (exit 1); exit 1; }; }
+		fi
 	else
 		LINUX_SYMBOLS=NONE
 	fi