configure: custom macro to check for library header

m4/knot-check-header.m4

@@ -0,0 +1,49 @@
+# KNOT_CHECK_HEADER([prefix], [name], [default], [header], [cflags], [libs])
+# -----------------------------------------------------------------------------
+# Check presence of a library by checking for a header file.
+#
+# - adds --enable-prefix configure flag
+#
+# - if $enable_prefix is yes or auto, checks for the header file
+#
+# - emits an error if $enable_prefix is yes and the header is not present
+#
+# - check can be overridden by setting prefix_CFLAGS and prefix_LIBS
+#   environment variables
+#
+# Output variables: $enable_foo (yes or no), $foo_CFLAGS, and $foo_LIBS
+#
+AC_DEFUN([KNOT_CHECK_HEADER],
+[
+  AC_ARG_ENABLE([$1], AC_HELP_STRING([--enable-$1], [Support for $2 [default $3]]),
+    [enable_][$1][=$enableval], [enable_][$1][=][$3])
+
+  AC_ARG_VAR([$1][_CFLAGS], [C compiler flags for $2, overriding defaults])
+  AC_ARG_VAR([$1][_LIBS], [linker flags for $2, overriding defaults])
+
+  AS_CASE([$enable_][$1],
+    [no], [
+      [$1][_CFLAGS]=
+      [$1][_LIBS]=
+    ],
+    [auto|yes], [
+      AS_IF([test -n "$][$1][_LIBS"], [
+        dnl: skip header check if environment variable is set
+        [enable_][$1][=yes]
+      ],[
+        dnl: check for header
+        AC_CHECK_HEADER([$4], [
+          [enable_][$1]=yes
+          [$1][_CFLAGS]=[$5]
+          [$1][_LIBS]=[$6]
+        ], [
+          AS_IF([test "$enable_][$1][" = auto],
+            [[enable_][$1]=no],
+            [AC_MSG_ERROR([Header file "$4" for $2 not found])]
+          )
+        ])
+      ])
+    ],
+    [AC_MSG_ERROR([Invalid value of --enable-$1])]
+  )
+])