chore: make libxml optional

DESCRIPTION

@@ -72,4 +72,4 @@ Config/testthat/start-first: vs-es, scan, vs-operators, weakref,
 Encoding: UTF-8
 Roxygen: list(markdown = TRUE)
 RoxygenNote: 7.3.1
-SystemRequirements: libxml2, glpk (>= 4.57, optional)
+SystemRequirements: libxml2 (optional), glpk (>= 4.57, optional)

configure

@@ -8,24 +8,28 @@ elif pkg-config --version >/dev/null 2>&1; then
     libxml2_cflags=$(pkg-config --cflags libxml-2.0)
     libxml2_libs=$(pkg-config --libs --static libxml-2.0)
 else
-    echo "Error: Neither xml2-config nor pkg-config is available"
-    exit 1
+    echo "Warning: libxml2 not found. Neither xml2-config nor pkg-config is available."
+    libxml_unavilable=true
 fi
 
 if [ -n "$libxml2_cflags" ]; then
-    PKG_CFLAGS="$PKG_CFLAGS $libxml2_cflags"
+    PKG_CFLAGS="$PKG_CFLAGS $libxml2_cflags -DHAVE_LIBXML"
     echo "libxml2 include directories: $libxml2_cflags"
 else
-    echo "Error: libxml2 include directory not found"
-    exit 1
+    echo "Warning: libxml2 include directory not found."
+    libxml_unavilable=true
 fi
 
 if [ -n "$libxml2_libs" ]; then
     PKG_LIBS="$PKG_LIBS $libxml2_libs"
     echo "libxml2 library link flags: $libxml2_libs"
 else
-    echo "Error: libxml2 library link flags not found"
-    exit 1
+    echo "Warning: libxml2 library link flags not found."
+    libxml_unavilable=true
+fi
+
+if [ "$libxml_unavilable" = true ]; then
+    echo "GraphML import will not be available."
 fi
 
 OBJECTS='${SOURCES} ${MINIGMPSOURCES}'

src/Makevars.ucrt

@@ -9,7 +9,8 @@ PKG_CPPFLAGS = -I"${LIB_XML}/include/libxml2" -I"${LIB_XML}/include" -DLIBXML_ST
     -DNDEBUG -DNTIMER -DNPRINT -DIGRAPH_THREAD_LOCAL= \
     -DPRPACK_IGRAPH_SUPPORT \
     -DHAVE_GFORTRAN=1 \
-    -D_GNU_SOURCE=1
+    -D_GNU_SOURCE=1 \
+    -DHAVE_LIBXML
 
 PKG_LIBS = -L"${LIB_XML}/lib" -lxml2 -liconv -lz -lws2_32 -lstdc++ \
   -lglpk $(BLAS_LIBS) $(LAPACK_LIBS) $(FLIBS) -llzma

src/Makevars.win

@@ -9,7 +9,8 @@ PKG_CPPFLAGS=-DUSING_R -I. -Ivendor -Ivendor/cigraph/src -Ivendor/cigraph/includ
     -DNDEBUG -DNTIMER -DNPRINT -DIGRAPH_THREAD_LOCAL= \
     -DPRPACK_IGRAPH_SUPPORT \
     -DHAVE_GFORTRAN=1 \
-    -D_GNU_SOURCE=1
+    -D_GNU_SOURCE=1 \
+    -DHAVE_LIBXML
 
 PKG_LIBS = -lxml2 -lz -lstdc++ \
   -lglpk $(BLAS_LIBS) $(LAPACK_LIBS)

src/vendor/config.h

@@ -22,7 +22,6 @@
 #endif
 
 #define HAVE_GLPK 1
-#define HAVE_LIBXML 1
 
 /* #undef INTERNAL_BLAS */
 /* #undef INTERNAL_LAPACK */