cmake: suggest -D arguments required by --find-package

--find-package (cmake's legacy pkg-config-like mode) silently requires
-DNAME=, -DLANGUAGE=, -DCOMPILER_ID= and -DMODE= to be set, or cmake
aborts with a chain of 'argument not specified' errors (see
Modules/CMakeFindPackageMode.cmake). The completion script gave no
hint that these were needed.

Detect --find-package on the command line and, when present, offer
NAME/LANGUAGE/COMPILER_ID/MODE alongside the usual -D property names,
plus value completion for each (e.g. LANGUAGE -> C/CXX/Fortran,
MODE -> EXIST/COMPILE/LINK).

Fixes #593

src/_cmake

@@ -614,6 +614,15 @@ _cmake_define_property_names() {
   local alternatives=(
     'common-property-names:common property name:_cmake_define_common_property_names -qS='
   )
+
+  # --find-package (cmake's legacy pkg-config-like mode) requires
+  # -DNAME=, -DLANGUAGE=, -DCOMPILER_ID= and -DMODE= to be set, see
+  # Modules/CMakeFindPackageMode.cmake. Suggest them whenever
+  # --find-package is present on the command line.
+  if (( ${words[(I)--find-package]} )); then
+    alternatives+=('find-package-property-names:find-package property name:_cmake_find_package_property_names -qS=')
+  fi
+
   local -A cmake_langs
   zstyle -a ":completion:${curcontext}:" languages cmake_langs
   [[ $#cmake_langs -eq 0 ]] && cmake_langs=('C' 'C' 'CXX' 'C++')
@@ -649,6 +658,21 @@ _cmake_define_lang_property_names() {
   _describe -t "${cmake_lang//:/-}-property-names" "${cmake_lang_desc} property name" properties $@[0,-3] && return 0
 }
 
+# ------------------------------------
+# _cmake_find_package_property_names
+# ------------------------------------
+(( $+functions[_cmake_find_package_property_names] )) ||
+_cmake_find_package_property_names() {
+  local -a properties=(
+    'NAME:Name of the package to search for, e.g. JPEG'
+    'LANGUAGE:Language context for the search (C, CXX or Fortran)'
+    'COMPILER_ID:Compiler identification to use, e.g. GNU'
+    'MODE:Search mode (EXIST, COMPILE or LINK)'
+  )
+
+  _describe -t 'find-package-property-names' 'find-package property name' properties "$@"
+}
+
 # -----------------------------------
 # _cmake_define_common_property_names
 # -----------------------------------
@@ -710,6 +734,19 @@ _cmake_define_property_values() {
     (CMAKE_UNITY_BUILD)
       _wanted booleans expl 'boolean' _cmake_booleans && ret=0
       ;;
+    (NAME)
+      _message -e package-name 'package name to search for, e.g. JPEG' && ret=0
+      ;;
+    (LANGUAGE)
+      _wanted languages expl 'language' _values 'language' 'C' 'CXX' 'Fortran' && ret=0
+      ;;
+    (COMPILER_ID)
+      _wanted compiler-ids expl 'compiler id' _values 'compiler id' \
+        'GNU' 'Clang' 'AppleClang' 'MSVC' 'Intel' 'IntelLLVM' 'PGI' 'XL' 'Cray' 'Fujitsu' 'SunPro' 'HP' 'Compaq' 'Borland' 'Watcom' && ret=0
+      ;;
+    (MODE)
+      _wanted modes expl 'find-package mode' _values 'mode' 'EXIST' 'COMPILE' 'LINK' && ret=0
+      ;;
     (CMAKE_INSTALL_PREFIX)
       _files -/ && ret=0
       ;;