Add support for M4 CXX flag detection in CMakeLists

Enables all flags that are detected by M4 builds to be detected by CMake builds
as well (except for -Wall which enables a bunch of junk).

(cherry picked from commit 2ea8a97)

infrastructure/cmake/CMakeLists.txt

@@ -480,7 +480,7 @@ move_file_if_exists(
 	"${boxconfig_cmake_h_dir}/BoxConfig.cmake.h.bak")
 
 foreach(m4_filename boxbackup_tests.m4 ax_check_mount_point.m4 ax_func_syscall.m4)
-	file(STRINGS "${base_dir}/infrastructure/m4/${m4_filename}" m4_functions REGEX "^ *AC[_A-Z]+\\(.*\\)$")
+	file(STRINGS "${base_dir}/infrastructure/m4/${m4_filename}" m4_functions REGEX "^ *(AC|AX|BOX)_[A-Z_]+\\(.*\\)$")
 	foreach(m4_function ${m4_functions})
 		if(DEBUG)
 			message(STATUS "Processing m4_function: ${m4_function}")
@@ -562,6 +562,22 @@ foreach(m4_filename boxbackup_tests.m4 ax_check_mount_point.m4 ax_func_syscall.m
 				}
 				]=] "HAVE_${platform_var_name}")
 			file(APPEND "${boxconfig_h_file}" "#cmakedefine HAVE_${platform_var_name}\n")
+		elseif(m4_function MATCHES "^ *BOX_CHECK_CXX_FLAG\\((-[A-Za-z_,=-]+)\\)")
+			if(DEBUG)
+				message(STATUS "Processing BOX_CHECK_CXX_FLAG: ${CMAKE_MATCH_1}")
+			endif()
+
+			if(NOT CMAKE_MATCH_1 STREQUAL "-Wall")
+				set(flag "${CMAKE_MATCH_1}")
+				string(TOLOWER "have_flag_${flag}" have_flag_var_name)
+				string(REGEX REPLACE "[^a-z_]" "_" have_flag_var_name ${have_flag_var_name})
+				string(REGEX REPLACE "__+" "_" have_flag_var_name ${have_flag_var_name})
+
+				CHECK_CXX_COMPILER_FLAG(${flag} ${have_flag_var_name})
+				if(${have_flag_var_name})
+					add_definitions("${flag}")
+				endif()
+			endif()
 		endif()
 	endforeach()
 

infrastructure/m4/boxbackup_tests.m4

@@ -10,17 +10,27 @@ solaris*)
   ;;
 esac
 
-# Use -Wall if the compiler supports it. This gives better warnings.
-AX_CHECK_COMPILE_FLAG(-Wall, [cxxflags_strict="$cxxflags_strict -Wall"])
-AX_CHECK_COMPILE_FLAG(-Wundef, [cxxflags_strict="$cxxflags_strict -Wundef"])
-AX_CHECK_COMPILE_FLAG(-Werror=return-type,
-	[cxxflags_strict="$cxxflags_strict -Werror=return-type"])
-AX_CHECK_COMPILE_FLAG(-Werror=non-virtual-dtor,
-	[cxxflags_strict="$cxxflags_strict -Werror=non-virtual-dtor"])
-AX_CHECK_COMPILE_FLAG(-Werror=delete-non-virtual-dtor,
-	[cxxflags_strict="$cxxflags_strict -Werror=delete-non-virtual-dtor"])
-AX_CHECK_COMPILE_FLAG(-Werror=narrowing,
-	[cxxflags_strict="$cxxflags_strict -Werror=narrowing"])
+# If the compiler supports it, force errors on unknown flags, so that detection works:
+AX_CHECK_COMPILE_FLAG(-Werror=unknown-warning-option,
+	[cxxflags_force_error="-Werror=unknown-warning-option"])
+
+# Reduce compiler flag checking to a one-liner, needed for CMake to parse them
+AC_DEFUN([BOX_CHECK_CXX_FLAG],
+	AX_CHECK_COMPILE_FLAG($1,
+		[cxxflags_strict="$cxxflags_strict $1"],,
+		$cxxflags_force_error)
+)
+
+# Enable some compiler flags if the compiler supports them. This gives better warnings
+# and detects some problems early.
+BOX_CHECK_CXX_FLAG(-Wall)
+BOX_CHECK_CXX_FLAG(-Werror=return-type)
+BOX_CHECK_CXX_FLAG(-Werror=non-virtual-dtor)
+BOX_CHECK_CXX_FLAG(-Werror=delete-non-virtual-dtor)
+# This error is detected by MSVC, but not usually by GCC/Clang:
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58114
+BOX_CHECK_CXX_FLAG(-Werror=delete-incomplete)
+
 AC_SUBST([CXXFLAGS_STRICT], [$cxxflags_strict])
 
 if test "x$GXX" = "xyes"; then