CMake: use find_program() to look for bash

This is to fail early, during the configuration step, and not later,
during the compilation step if bash is not installed.
Compile tested with and without bash installed on OpenBSD 7.5 amd64.

Author: guijan

src/cbmc/CMakeLists.txt

@@ -57,8 +57,9 @@ endif()
 
 # bash completion
 if(NOT WIN32)
+  find_program(BASH_PROGRAM bash REQUIRED)
   add_custom_command(OUTPUT "${CBMC_ROOT_DIR}/scripts/bash-autocomplete/cbmc.sh"
-    COMMAND "${CBMC_ROOT_DIR}/scripts/bash-autocomplete/extract_switches.sh" "${CMAKE_CXX_COMPILER}"
+    COMMAND "${BASH_PROGRAM}" "${CBMC_ROOT_DIR}/scripts/bash-autocomplete/extract_switches.sh" "${CMAKE_CXX_COMPILER}"
     DEPENDS $<TARGET_FILE:cbmc>
   )
   add_custom_target(cbmc.sh ALL