Add version detection from fftw-wisdom-to-conf when pkg-config is not used

When `pkg-config` is not found or `FFTW_ROOT` is set, the version of FFTW was not determined. This commit adds a fallback that attempts to extract the version from the `fftw-wisdom-to-conf` binary using its `-V` flag. If the command fails, a warning is issued with the output and error messages.

The assignment of `FFTW_VERSION` is now conditional: it is set from `pkg-config` results only when available, and otherwise from the fallback method. This ensures the version variable is only populated when a reliable source is found.

Author: zhao-shihan

FindFFTW.cmake

@@ -45,14 +45,33 @@ endif()
 # Check if we can use PkgConfig
 find_package(PkgConfig)
 
-#Determine from PKG
+# Determine from PKG
 if( PKG_CONFIG_FOUND AND NOT FFTW_ROOT )
     pkg_check_modules( PKG_FFTW QUIET "fftw3" )
+    set( FFTW_VERSION ${PKG_FFTW_VERSION} )
+else()
+    # If pkg-config was skipped, there seems no way to get the version directly.
+    # Try to deduce the version from fftw-wisdom-to-conf instead.
+    #   (From @kprussing. See https://github.com/egpbos/findFFTW/pull/8)
+    execute_process(COMMAND ${FFTW_ROOT}/bin/fftw-wisdom-to-conf -V
+                    RESULT_VARIABLE _fftw_wtc_success
+                    OUTPUT_VARIABLE _fftw_wtc_stdout
+                    ERROR_VARIABLE _fftw_wtc_stderr)
+    if (_fftw_wtc_success EQUAL 0)
+        string(REGEX MATCH "FFTW *version *([0-9]+([.][0-9]+([.][0-9]+)?)?)"
+                _fftw_wtc_version ${_fftw_wtc_stdout})
+        string(REPLACE " " ";" _fftw_wtc_version_list ${_fftw_wtc_version})
+        list(GET _fftw_wtc_version_list -1 FFTW_VERSION)
+    else()
+        message(WARNING "Error running ${FFTW_ROOT}/bin/fftw-wisdom-to-conf. "
+                        "Could not determine FFTW version.
+Output:
+${_fftw_wtc_stdout}
+Error:
+${_fftw_wtc_stderr}")
+    endif()
 endif()
 
-# Set variables based on pkg-config results
-set(FFTW_VERSION ${PKG_FFTW_VERSION})
-
 #Check whether to search static or dynamic libs
 set( CMAKE_FIND_LIBRARY_SUFFIXES_SAV ${CMAKE_FIND_LIBRARY_SUFFIXES} )