Build: Let the users override the symbol versioning variant.

There are cases when the users want to decide themselves whether
they want to have the generic (even on GNU/Linux) or the linux
(even if we do not recommend that) symbol versioning variant.
The former might be needed to circumvent compiler issues (i.e.
the compiler does not support all features that are required
for the linux versioning), the latter might help in overriding
the assumptions made in the configure script.

(This commit was contributed under 0BSD but the author confirmed
that it is fine to backport it to the public domain branches. See
https://github.com/tukaani-project/xz/pull/90#issuecomment-2100185936
and the next two messages.)

(cherry picked from commit f56ed6fac6)

configure.ac

@@ -714,6 +714,7 @@ elif test "x$enable_shared" = xno ; then
 	enable_symbol_versions=no
 	AC_MSG_RESULT([no (not building a shared library)])
 else
+	if test "x$enable_symbol_versions" = xyes ; then
 		case "$host_cpu-$host_os" in
 			microblaze*)
 				# GCC 12 on MicroBlaze doesn't support __symver__
@@ -724,6 +725,15 @@ else
 				enable_symbol_versions=generic
 				;;
 			*-linux*)
+				enable_symbol_versions=linux
+				;;
+			*)
+				enable_symbol_versions=generic
+				;;
+		esac
+	fi
+
+	if test "x$enable_symbol_versions" = xlinux ; then
 		case "$pic_mode-$enable_static" in
 			default-*)
 				# Use symvers if PIC is defined.
@@ -744,7 +754,6 @@ else
     possibly picking only src/liblzma/.libs/liblzma.a from the static build.])
 				;;
 		esac
-			enable_symbol_versions=linux
 		AC_DEFINE_UNQUOTED([HAVE_SYMBOL_VERSIONS_LINUX],
 			[$have_symbol_versions_linux],
 			[Define to 1 to if GNU/Linux-specific details
@@ -755,11 +764,11 @@ else
 			time with Libtool if neither --with-pic nor
 			--without-pic is used). This define must be
 			used together with liblzma_linux.map.])
-			;;
-		*)
-			enable_symbol_versions=generic
-			;;
-	esac
+	elif test "x$enable_symbol_versions" != xgeneric ; then
+		AC_MSG_RESULT([])
+		AC_MSG_ERROR(
+			[unknown symbol versioning variant '$enable_symbol_versions'])
+	fi
 	AC_MSG_RESULT([yes ($enable_symbol_versions)])
 fi