Link the main library with -Wl,-z,nodelete to prevent unloading (#2934)

See: https://github.com/libvips/php-vips-ext/pull/44

configure.ac

@@ -457,6 +457,20 @@ else
   fi
 fi
 
+SAVE_LDFLAGS="$LDFLAGS"
+LDFLAGS="$LDFLAGS -Wl,-z,nodelete"
+AC_MSG_CHECKING([whether linker understands -z nodelete])
+AC_LINK_IFELSE([AC_LANG_PROGRAM([], [])],
+  [
+    AC_MSG_RESULT([yes])
+    LDFLAGS_Z_NODELETE="-Wl,-z,nodelete"
+  ],[
+    AC_MSG_RESULT([no])
+    LDFLAGS_Z_NODELETE=""
+  ])
+LDFLAGS="$SAVE_LDFLAGS"
+AC_SUBST(LDFLAGS_Z_NODELETE)
+
 # check for gtk-doc
 GTK_DOC_CHECK([1.14],[--flavour no-tmpl])
 

libvips/Makefile.am

@@ -58,6 +58,7 @@ libvips_la_LIBADD = \
 	@VIPS_LIBS@
 
 libvips_la_LDFLAGS = \
+	$(LDFLAGS_Z_NODELETE) \
 	-no-undefined \
 	-version-info @LIBRARY_CURRENT@:@LIBRARY_REVISION@:@LIBRARY_AGE@ 
 

libvips/meson.build

@@ -26,6 +26,7 @@ libvips_lib = library('vips',
     darwin_versions: darwin_versions,
     gnu_symbol_visibility: 'hidden',
     install: true,
+    link_args: nodelete_link_args,
 )
 
 libvips_dep = declare_dependency(

meson.build

@@ -67,6 +67,7 @@ libvips_deps = [
     thread_dep,
     m_dep,
 ]
+nodelete_link_args = cc.get_supported_link_arguments('-Wl,-z,nodelete')
 
 prefix_dir = get_option('prefix')
 lib_dir = prefix_dir / get_option('libdir')