config: better libtirpc detection

Improve the autoconf code for finding libtirpc and do not assume the
headers are in /usr/include/tirpc.

Also remove this assumption from the `rpc/xdr.h` header in libspl and
use the same `#include_next` mechanism that is used for other libspl
headers.

Include pkg.m4 from pkg-config in config/ for PKG_CHECK_MODULES(), the
file license allows this.

Include ax_save_flags.m4 and ax_restore_flags.m4 from autoconf-archive,
the file licenses are compatible. Use the 2012 versions so as not rely
on a more recent autoconf feature AS_VAR_COPY(), which breaks some build
slaves.

Add new macro library `config/find_system_library.m4` which defines the
FIND_SYSTEM_LIBRARY() macro which is a convenience wrapper over using
PKG_CHECK_MODULES() with a fallback to standard library locations and
some sanity checks.

The parameters are:

```
FIND_SYSTEM_LIBRARY(VARIABLE-PREFIX, MODULE, HEADER, HEADER-PREFIXES,
    LIBRARY, FUNCTIONS, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])
```

`HEADER-PREFIXES` and `FUNCTIONS` are comma-separated m4 lists.

For libtirpc we are using:

```
FIND_SYSTEM_LIBRARY(LIBTIRPC, [libtirpc], [rpc/xdr.h], [tirpc], [tirpc],
    [xdrmem_create], [], [...])
```

The headers are first checked for without the prefixes and then with.

This system works with pkg-config and falls back on checking standard
header/library locations, it can be easily overridden by the user by
setting the `PREFIX_CFLAGS` and `PREFIX_LIBS` variables which are
automatically added to the `./configure --help` output.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Rafael Kitover <[email protected]>
Closes #7422 
Closes #8313 

1 files changed, 31 insertions, 0 deletions

diff --git a/config/ax_restore_flags.m4 b/config/ax_restore_flags.m4
new file mode 100644
index 000000000..cf03cae79
--- /dev/null
+++ b/config/ax_restore_flags.m4
@@ -0,0 +1,31 @@
+# ===========================================================================
+#     http://www.gnu.org/software/autoconf-archive/ax_restore_flags.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_RESTORE_FLAGS()
+#
+# DESCRIPTION
+#
+#   Restore common compilation flags from temporary variables
+#
+# LICENSE
+#
+#   Copyright (c) 2009 Filippo Giunchedi <[email protected]>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved. This file is offered as-is, without any
+#   warranty.
+
+#serial 3
+
+AC_DEFUN([AX_RESTORE_FLAGS], [
+  CPPFLAGS="${CPPFLAGS_save}"
+  CFLAGS="${CFLAGS_save}"
+  CXXFLAGS="${CXXFLAGS_save}"
+  OBJCFLAGS="${OBJCFLAGS_save}"
+  LDFLAGS="${LDFLAGS_save}"
+  LIBS="${LIBS_save}"
+])