Respect --sysconfdir in source files

* Prefix installations need to point to a non-root `etc`
  - Gentoo Prefix has been patching this for over 10 years:
    https://bugs.gentoo.org/317891
  - MacPorts has to manually replace paths after patching:
    cc3bb736e9/textproc/libxml2/Portfile (L46)

CMakeLists.txt

@@ -361,6 +361,8 @@ endif()
 add_library(LibXml2 ${LIBXML2_HDRS} ${LIBXML2_SRCS})
 add_library(LibXml2::LibXml2 ALIAS LibXml2)
 
+target_compile_definitions(LibXml2 PRIVATE SYSCONFDIR="${CMAKE_INSTALL_FULL_SYSCONFDIR}")
+
 if(NOT BUILD_SHARED_LIBS)
 	target_compile_definitions(LibXml2 INTERFACE LIBXML_STATIC)
 	set(XML_CFLAGS "-DLIBXML_STATIC")
@@ -484,6 +486,7 @@ if(LIBXML2_WITH_PROGRAMS)
 	foreach(PROGRAM ${PROGRAMS})
 		add_executable(${PROGRAM} ${PROGRAM}.c)
 		add_executable(LibXml2::${PROGRAM} ALIAS ${PROGRAM})
+		target_compile_definitions(${PROGRAM} PRIVATE SYSCONFDIR="${CMAKE_INSTALL_FULL_SYSCONFDIR}")
 		target_link_libraries(${PROGRAM} LibXml2)
 		if(HAVE_LIBHISTORY)
 			target_link_libraries(${PROGRAM} history)
@@ -539,6 +542,7 @@ if(LIBXML2_WITH_TESTS)
 		)
 		foreach(TEST ${TESTS_THREADS})
 			add_executable(${TEST} ${TEST}.c)
+			target_compile_definitions(${TEST} PRIVATE SYSCONFDIR="${CMAKE_INSTALL_FULL_SYSCONFDIR}")
 			if(WIN32)
 				target_compile_definitions(${TEST} PRIVATE HAVE_WIN32_THREADS)
 			endif()

Makefile.am

@@ -9,7 +9,7 @@ endif
 
 DIST_SUBDIRS = include . doc example fuzz python xstc
 
-AM_CPPFLAGS = -I$(top_builddir)/include -I$(srcdir)/include
+AM_CPPFLAGS = -I$(top_builddir)/include -I$(srcdir)/include -DSYSCONFDIR='"$(sysconfdir)"'
 
 AM_CFLAGS = $(EXTRA_CFLAGS) $(THREAD_CFLAGS) $(Z_CFLAGS) $(LZMA_CFLAGS)
 

catalog.c

@@ -68,15 +68,15 @@
 #define XML_URN_PUBID "urn:publicid:"
 #define XML_CATAL_BREAK ((xmlChar *) -1)
 #ifndef XML_XML_DEFAULT_CATALOG
-#define XML_XML_DEFAULT_CATALOG "file:///etc/xml/catalog"
+#define XML_XML_DEFAULT_CATALOG "file://" SYSCONFDIR "/xml/catalog"
 #endif
 #ifndef XML_SGML_DEFAULT_CATALOG
-#define XML_SGML_DEFAULT_CATALOG "file:///etc/sgml/catalog"
+#define XML_SGML_DEFAULT_CATALOG "file://" SYSCONFDIR "/sgml/catalog"
 #endif
 
 #if defined(_WIN32) && defined(_MSC_VER)
 #undef XML_XML_DEFAULT_CATALOG
-static char XML_XML_DEFAULT_CATALOG[256] = "file:///etc/xml/catalog";
+static char XML_XML_DEFAULT_CATALOG[256] = "file://" SYSCONFDIR "/xml/catalog";
 #if !defined(_WINDOWS_)
 void* __stdcall GetModuleHandleA(const char*);
 unsigned long __stdcall GetModuleFileNameA(void*, char*, unsigned long);

libxml.h

@@ -28,6 +28,10 @@
 #include "config.h"
 #include <libxml/xmlversion.h>
 
+#ifndef SYSCONFDIR
+  #define SYSCONFDIR "/etc"
+#endif
+
 #if defined(__Lynx__)
 #include <stdio.h> /* pull definition of size_t */
 #include <varargs.h>

runtest.c

@@ -2927,7 +2927,7 @@ static int urip_rlen;
  */
 static int
 uripMatch(const char * URI) {
-    if ((URI == NULL) || (!strcmp(URI, "file:///etc/xml/catalog")))
+    if ((URI == NULL) || (!strcmp(URI, "file://" SYSCONFDIR "/xml/catalog")))
         return(0);
     /* Verify we received the escaped URL */
     if (strcmp(urip_rcvsURLs[urip_current], URI))
@@ -2946,7 +2946,7 @@ uripMatch(const char * URI) {
  */
 static void *
 uripOpen(const char * URI) {
-    if ((URI == NULL) || (!strcmp(URI, "file:///etc/xml/catalog")))
+    if ((URI == NULL) || (!strcmp(URI, "file://" SYSCONFDIR "/xml/catalog")))
         return(NULL);
     /* Verify we received the escaped URL */
     if (strcmp(urip_rcvsURLs[urip_current], URI))

xmlcatalog.c

@@ -40,7 +40,7 @@ static char *filename = NULL;
 
 
 #ifndef XML_SGML_DEFAULT_CATALOG
-#define XML_SGML_DEFAULT_CATALOG "/etc/sgml/catalog"
+#define XML_SGML_DEFAULT_CATALOG SYSCONFDIR "/sgml/catalog"
 #endif
 
 /************************************************************************

xmllint.c

@@ -81,7 +81,7 @@
 #endif
 
 #ifndef XML_XML_DEFAULT_CATALOG
-#define XML_XML_DEFAULT_CATALOG "file:///etc/xml/catalog"
+#define XML_XML_DEFAULT_CATALOG "file://" SYSCONFDIR "/xml/catalog"
 #endif
 
 typedef enum {