diff --git a/acconfig.h b/acconfig.h
index bea917394..6c852f057 100644
--- a/acconfig.h
+++ b/acconfig.h
@@ -100,6 +100,12 @@
 /* Define this if fnmatch is prototyped in fnmatch.h */
 #undef HAVE_FNMATCH_PROTO
 
+/* Define this if strnlen is prototyped in string.h */
+#undef HAVE_STRNLEN_PROTO
+
+/* Define this if fnmatch is prototyped in string.h */
+#undef HAVE_STRCASESTR_PROTO
+
 /* Define this to something appropriate for declaring 0 length arrays */
 #undef ZERO_LENGTH_ARRAY
 
diff --git a/configure.ac b/configure.ac
index dc5af4fbc..1813fb3b5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -223,6 +223,24 @@ AC_TRY_COMPILE(
 	AC_MSG_RESULT(no)
 )
 
+AC_MSG_CHECKING(for strnlen in string.h)
+AC_TRY_COMPILE(
+	[#include "string.h"],
+	[int (*foo)() = strnlen;],
+	AC_DEFINE(HAVE_STRNLEN_PROTO)
+	AC_MSG_RESULT(yes),
+	AC_MSG_RESULT(no)
+)
+
+AC_MSG_CHECKING(for strcasestr in string.h)
+AC_TRY_COMPILE(
+	[#include "string.h"],
+	[int (*foo)() = strnlen;],
+	AC_DEFINE(HAVE_STRCASESTR_PROTO)
+	AC_MSG_RESULT(yes),
+	AC_MSG_RESULT(no)
+)
+
 AC_MSG_CHECKING(for type of fpos_t)
 AC_TRY_COMPILE(
 	[#include <stdio.h>],
@@ -307,8 +325,8 @@ AC_FUNC_VPRINTF
 AC_CHECK_FUNCS(
 	access _access \
 	gethostname gethostbyname connect gettimeofday getwd mkdir _mkdir \
-	ftime _ftime fcntl stat putenv select socket strerror strstr \
-	snprintf _snprintf vsnprintf _vsnprintf strsep dlopen getaddrinfo \
+	ftime _ftime fcntl stat putenv select socket strerror strcasestr strnlen \
+	strstr snprintf _snprintf vsnprintf _vsnprintf strsep dlopen getaddrinfo \
 	getnameinfo mprotect
 )
 
diff --git a/include/compat.h b/include/compat.h
index 1024a5ccb..cf5551b6a 100644
--- a/include/compat.h
+++ b/include/compat.h
@@ -91,13 +91,19 @@ extern int vsnprintf(char *s, size_t maxlen, const char *format, va_list arg);
 #if !defined(strncaseequal)
 # define strncaseequal(a,b,c) (strncasecmp (a, b, c) == 0)
 #endif
-// FIXME: glibc has strcasestr, but only declares it if __USE_GNU is defined
-#if !defined(strcasestr)
+#ifdef HAVE_STRCASESTR
+# ifndef HAVE_STRCASESTR_PROTO
+extern char *strcasestr (const char *__haystack, const char *__needle);
+# endif
+#else
 # define strcasestr Q_strcasestr
 # define need_qstring_h
 #endif
-// FIXME: same as above
-#if !defined(strnlen)
+#ifdef HAVE_STRNLEN
+# ifndef HAVE_STRNLEN_PROTO
+size_t strnlen (const char *str, size_t len);
+# endif
+#else
 # define strnlen Q_strnlen
 # define need_qstring_h
 #endif