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 

3 files changed, 8 insertions, 17 deletions

diff --git a/lib/libnvpair/Makefile.am b/lib/libnvpair/Makefile.am
index 8d6519f30..6d59d7bfc 100644
--- a/lib/libnvpair/Makefile.am
+++ b/lib/libnvpair/Makefile.am
@@ -4,11 +4,9 @@ VPATH = \
 	$(top_srcdir)/module/nvpair \
 	$(top_srcdir)/lib/libnvpair
 
-# Required CFLAGS for libtirpc
-AM_CFLAGS += $(LIBTIRPC_CFLAGS)
-
 # Includes kernel code, generate warnings for large stack frames
-AM_CFLAGS += $(FRAME_LARGER_THAN)
+# and required CFLAGS for libtirpc
+AM_CFLAGS += $(FRAME_LARGER_THAN) $(LIBTIRPC_CFLAGS)
 
 DEFAULT_INCLUDES += \
 	-I$(top_srcdir)/include \
@@ -30,7 +28,7 @@ nodist_libnvpair_la_SOURCES = \
 	$(USER_C) \
 	$(KERNEL_C)
 
-libnvpair_la_LIBADD = $(LIBTIRPC)
+libnvpair_la_LIBADD = $(LIBTIRPC_LIBS)
 libnvpair_la_LDFLAGS = -version-info 1:1:0
 
 EXTRA_DIST = $(USER_C)
diff --git a/lib/libspl/Makefile.am b/lib/libspl/Makefile.am
index 5fceaf6a5..cd74676dd 100644
--- a/lib/libspl/Makefile.am
+++ b/lib/libspl/Makefile.am
@@ -10,6 +10,8 @@ DIST_SUBDIRS = include asm-generic asm-i386 asm-x86_64
 DEFAULT_INCLUDES += \
 	-I$(top_srcdir)/lib/libspl/include
 
+AM_CFLAGS += $(LIBTIRPC_CFLAGS)
+
 AM_CCASFLAGS = \
 	$(CFLAGS)
 
@@ -35,6 +37,6 @@ nodist_libspl_la_SOURCES = \
 	$(USER_C) \
 	$(USER_ASM)
 
-libspl_la_LIBADD = -lrt
+libspl_la_LIBADD = -lrt $(LIBTIRPC_LIBS)
 
 EXTRA_DIST = $(USER_C)
diff --git a/lib/libspl/include/rpc/xdr.h b/lib/libspl/include/rpc/xdr.h
index 3c7c53fcc..27e4395c7 100644
--- a/lib/libspl/include/rpc/xdr.h
+++ b/lib/libspl/include/rpc/xdr.h
@@ -32,21 +32,12 @@
 #ifndef LIBSPL_RPC_XDR_H
 #define	LIBSPL_RPC_XDR_H
 
-/*
- * When available prefer libtirpc for xdr functionality.  This library is
- * mandatory when compiling with musl libc because it does not provide xdr.
- */
-#if defined(HAVE_LIBTIRPC)
+#include_next <rpc/xdr.h>
 
-#include <tirpc/rpc/xdr.h>
-#ifdef xdr_control
+#ifdef xdr_control /* if e.g. using tirpc */
 #undef xdr_control
 #endif
 
-#else
-#include_next <rpc/xdr.h>
-#endif /* HAVE_LIBTIRPC */
-
 #define	XDR_GET_BYTES_AVAIL 1
 
 typedef struct xdr_bytesrec {