autogen.sh: Also warn if AX_CXX_COMPILE_STDCXX_11 is not available.

This fixes an error message when the file is not installed at all:

  ./autogen.sh: 67: [: Illegal number:

Turn the AX_CXX_COMPILE_STDCXX_11 errors into warnings though, since
it is (and should be) possible to build libsigrok (just the C library)
just fine, even without that macro or with an older version of it.

diff --git a/autogen.sh b/autogen.sh
index 79cec4c293580ee10a2961d7825c2887617d75b2..8685c3c8f7cabd2822a7e9a6468895443ff82a98 100755 (executable)
--- a/autogen.sh
+++ b/autogen.sh
@@ -55,15 +55,19 @@ echo "Generating build system..."
 ${LIBTOOLIZE} --install --copy --quiet || exit 1
 aclocal ${ACLOCAL_DIR} || exit 1
 
-# check the version of a specific autoconf macro that tends to cause problems
+# Check the version of a specific autoconf macro that tends to cause problems.
 CXXMACROVERSION=$(
        grep -B 5 'm4_define(\[_AX_CXX_COMPILE_STDCXX_11_testbody\]' aclocal.m4 |
        sed -nr 's/.*serial[[:space:]]+([[:digit:]]+).*/\1/p'
 )
-if [ "$CXXMACROVERSION" -lt 4 ]; then
-       echo "error: the AX_CXX_COMPILE_STDCXX_11 macro on this system is too old."
-       echo "(found version $CXXMACROVERSION, at least 4 is required)"
-       exit 1
+if [ "x$CXXMACROVERSION" = "x" ]; then
+       echo "--- Warning: AX_CXX_COMPILE_STDCXX_11 macro not found."
+       echo "--- You won't be able to build the language bindings!"
+fi
+if [ "x$CXXMACROVERSION" != "x" ] && [ "$CXXMACROVERSION" -lt 4 ]; then
+       echo "--- Warning: AX_CXX_COMPILE_STDCXX_11 macro is too old."
+       echo "--- (found version $CXXMACROVERSION, at least 4 is required)"
+       echo "--- You won't be able to build the language bindings!"
 fi
 
 autoheader || exit 1