use PKG_CHECK_MODULES for fdk-aac

https://github.com/nu774/fdkaac/issues/60

Makefile.am

@@ -22,8 +22,10 @@ fdkaac_SOURCES = \
 
 dist_man_MANS = man/fdkaac.1
 
+fdkaac_CFLAGS = @CFLAGS@ @FDK_AAC_CFLAGS@
+
 fdkaac_LDADD = \
-    @LIBICONV@ @CHARSET_LIB@ -lfdk-aac -lm
+    @LIBICONV@ @CHARSET_LIB@ @FDK_AAC_LIBS@ -lm
 
 .rc.o:
 	$(RC) $< -o $@

configure.ac

@@ -16,8 +16,7 @@ AC_CHECK_TOOL(RC, windres,)
 
 AC_CHECK_HEADERS([sys/time.h])
 AC_CHECK_HEADERS([libcharset.h langinfo.h endian.h byteswap.h])
-AC_CHECK_HEADERS([fdk-aac/aacenc_lib.h], ,
-                 AC_MSG_ERROR([libfdk-aac is required]))
+PKG_CHECK_MODULES([FDK_AAC],[fdk-aac])
 
 AC_C_INLINE
 AC_C_BIGENDIAN