cmake: let the argp probe pass where argp-standalone needs strchrnul

On macOS with Homebrew's argp-standalone, Findargp's link test for
argp_parse fails with an undefined strchrnul, because libargp.a itself
references that GNU extension and the platform has none. The module then
stops with "does not have a symbol named argp_parse", which is not what
is wrong.

Probe for strchrnul first (with _GNU_SOURCE, so glibc answers correctly).
Where it exists, the test is unchanged. Where it is missing, run the same
argp_parse test with a local definition of strchrnul, and have ptouch-print
supply that definition for the real build.

On glibc nothing changes: the probe finds strchrnul and the original test
runs. Checked on Debian stable in the same session.

CMakeLists.txt

@@ -15,6 +15,11 @@ find_package(Git REQUIRED)
 find_package(PkgConfig REQUIRED)
 find_package(Intl REQUIRED)
 find_package(argp REQUIRED)
+if(NOT ARGP_IN_LIBC AND NOT HAVE_STRCHRNUL)
+	# See cmake/Findargp.cmake: argp-standalone needs a strchrnul() the
+	# platform does not have, so ptouch-print.c defines one.
+	add_compile_definitions(NEED_STRCHRNUL=1)
+endif()
 
 pkg_check_modules(LIBUSB REQUIRED libusb-1.0)
 

cmake/Findargp.cmake

@@ -62,9 +62,33 @@ if (ARGP_INCLUDE_PATH)
 
     # Check if argp_parse is available. Some implementations don't have this
     # symbol defined, thus they're not compatible.
+    #
+    # argp-standalone itself calls strchrnul(), a GNU extension that the
+    # platforms needing argp-standalone tend to lack (macOS among them). On
+    # those the plain link test fails with an undefined strchrnul and would
+    # report a perfectly good libargp as lacking argp_parse. So probe for
+    # strchrnul first and, where it is missing, run the argp_parse test with
+    # a local definition; the project then supplies the same definition.
     if (ARGP_LIBRARIES)
         set(CMAKE_REQUIRED_LIBRARIES "${ARGP_LIBRARIES}")
-        check_function_exists("argp_parse" ARGP_EXTERNAL)
+        include(CheckSymbolExists)
+        set(CMAKE_REQUIRED_DEFINITIONS -D_GNU_SOURCE)
+        check_symbol_exists("strchrnul" "string.h" HAVE_STRCHRNUL)
+        unset(CMAKE_REQUIRED_DEFINITIONS)
+        if (HAVE_STRCHRNUL)
+            check_function_exists("argp_parse" ARGP_EXTERNAL)
+        else ()
+            include(CheckCSourceCompiles)
+            check_c_source_compiles("
+                #include <string.h>
+                #include <argp.h>
+                char *strchrnul(const char *s, int c) {
+                    const char *p = strchr(s, c);
+                    return (char *)(p ? p : s + strlen(s));
+                }
+                int main(void) { return argp_parse(0, 1, (char *[]){\"x\"}, 0, 0, 0); }
+            " ARGP_EXTERNAL)
+        endif ()
         if (NOT ARGP_EXTERNAL)
             message(FATAL_ERROR "Your system ships an argp library in "
                     "${ARGP_LIBRARIES}, but it does not have a symbol "

src/ptouch-print.c

@@ -35,6 +35,16 @@
 
 #define _(s) gettext(s)
 
+#ifdef NEED_STRCHRNUL
+/* GNU extension. argp-standalone references it, so platforms that need
+   argp-standalone need this too. Selected by cmake/Findargp.cmake. */
+char *strchrnul(const char *s, int c)
+{
+	const char *p = strchr(s, c);
+	return (char *)(p ? p : s + strlen(s));
+}
+#endif
+
 #define MAX_LINES 4	/* this should be calculated depending on tape size */
 
 #define P_NAME "ptouch-print"