autotools: Abort if external libraries couldn't be found

This makes the configuration process more robust and deterministic.
Before, we would silently disable options.

configure.ac

@@ -751,6 +751,10 @@ if test "$with_modules" != "no" ; then
                 AC_DEFINE([HAVE_SHLLOAD], [], [Have shl_load based dso])])])
         LIBS=$_libs
     fi
+
+    if test "$WITH_MODULES" = "0"; then
+        AC_MSG_ERROR([dlopen or equivalent not found])
+    fi
 fi
 
 AC_SUBST(WITH_MODULES)
@@ -789,6 +793,10 @@ else
             ;;
     esac
 
+    if test "$WITH_THREADS" = "0"; then
+        AC_MSG_ERROR([libpthread not found])
+    fi
+
     case $host_os in
         *linux*)
             if test "${GCC}" = "yes" ; then
@@ -896,6 +904,10 @@ if test "$with_zlib" != "no" && test "$with_zlib" != ""; then
         LIBS=$_libs
     fi
 
+    if test "$WITH_ZLIB" = "0"; then
+        AC_MSG_ERROR([zlib not found])
+    fi
+
     XML_PRIVATE_CFLAGS="${XML_PRIVATE_CFLAGS} ${Z_CFLAGS}"
     XML_PRIVATE_LIBS="${XML_PRIVATE_LIBS} ${Z_LIBS}"
 fi
@@ -946,6 +958,10 @@ if test "$with_lzma" != "no" && test "$with_lzma" != ""; then
         LIBS=$_libs
     fi
 
+    if test "$WITH_LZMA" = "0"; then
+        AC_MSG_ERROR([liblzma not found])
+    fi
+
     XML_PRIVATE_CFLAGS="${XML_PRIVATE_CFLAGS} ${LZMA_CFLAGS}"
     XML_PRIVATE_LIBS="${XML_PRIVATE_LIBS} ${LZMA_LIBS}"
 fi
@@ -985,6 +1001,11 @@ else
             AC_MSG_RESULT([no])
         ])
     ])
+
+    if test "$WITH_ICONV" = "0"; then
+        AC_MSG_ERROR([libiconv not found])
+    fi
+
     if test "$WITH_ICONV" = "1" && test "$ICONV_DIR" != ""; then
         ICONV_CFLAGS="-I$ICONV_DIR/include"
         ICONV_LIBS="-L$ICONV_DIR/lib $ICONV_LIBS"
@@ -1046,6 +1067,10 @@ if test "$with_icu" != "no" && test "$with_icu" != "" ; then
         fi
     fi
 
+    if test "$WITH_ICU" = "0"; then
+        AC_MSG_ERROR([ICU not found])
+    fi
+
     XML_PRIVATE_CFLAGS="${XML_PRIVATE_CFLAGS} ${ICU_CFLAGS}"
     XML_PRIVATE_LIBS="${XML_PRIVATE_LIBS} ${ICU_LIBS}"
 fi