bash-completion: Avoid using _init_completion when not defined

With the current version of homebrew one gets:

   bash: _init_completion: command not found

Avoid this by initializing manually when the function is not defined.

Auxiliary/bash-completion/cmake

@@ -3,7 +3,14 @@
 _cmake()
 {
     local cur prev words cword split=false
-    _init_completion -n := || return
+    if type -t _init_completion >/dev/null; then
+        _init_completion -n = || return
+    else
+        # manual initialization for older bash completion versions
+        COMPREPLY=()
+        cur="${COMP_WORDS[COMP_CWORD]}"
+        prev="${COMP_WORDS[COMP_CWORD-1]}"
+    fi
 
     # Workaround for options like -DCMAKE_BUILD_TYPE=Release
     local prefix=

Auxiliary/bash-completion/cpack

@@ -3,7 +3,14 @@
 _cpack()
 {
     local cur prev words cword
+    if type -t _init_completion >/dev/null; then
         _init_completion -n = || return
+    else
+        # manual initialization for older bash completion versions
+        COMPREPLY=()
+        cur="${COMP_WORDS[COMP_CWORD]}"
+        prev="${COMP_WORDS[COMP_CWORD-1]}"
+    fi
 
     case "$prev" in
         -G)

Auxiliary/bash-completion/ctest

@@ -3,7 +3,14 @@
 _ctest()
 {
     local cur prev words cword
+    if type -t _init_completion >/dev/null; then
         _init_completion -n = || return
+    else
+        # manual initialization for older bash completion versions
+        COMPREPLY=()
+        cur="${COMP_WORDS[COMP_CWORD]}"
+        prev="${COMP_WORDS[COMP_CWORD-1]}"
+    fi
 
     case "$prev" in
         -C|--build-config)