commit 61a6af25d221bf790ac8f67ebfd88948f92babe4
Author:     Rodrigo Rebello <rprebello@gmail.com>
AuthorDate: Thu Oct 22 15:28:11 2015 -0200
Commit:     Rodrigo Rebello <rprebello@gmail.com>
CommitDate: Mon Oct 26 12:55:45 2015 -0200

    configure: fix --with-PACKAGE option checks

    Options of the form --with-PACKAGE[=yes] (e.g. --with-libpng), when
    passed to configure, were being treated as though --without-PACKAGE had
    been given.

    Although the intention is to have configure check and use PACKAGE by
    default if it's available, thus requiring the user to pass an option
    only if PACKAGE must NOT be used, there are times when the opposite
    might be desired (i.e. the user wants to indicate PACKAGE MUST be used).
    Moreover, allowing --with-PACKAGE and behaving as if --without-PACKAGE
    had been specified is in itself quite confusing.

    Fix that by testing the result of 'with_PACKAGE' in configure.ac and
    acting accordingly instead of blindly assuming a 'no'.
---
 configure.ac | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index ee97fe22..7b034853 100644
--- a/configure.ac
+++ b/configure.ac
@@ -80,7 +80,14 @@ AC_SDL_MAIN_WORKAROUND([
         [Build without libsamplerate @<:@default=check@:>@]),
     [],
     [
-        AC_CHECK_LIB(samplerate, src_new)
+        [with_libsamplerate=check]
+    ])
+    AS_IF([test "x$with_libsamplerate" != xno], [
+        AC_CHECK_LIB(samplerate, src_new, [], [
+            AS_IF([test "x$with_libsamplerate" != xcheck], [AC_MSG_FAILURE(
+                [--with-libsamplerate was given, but test for libsamplerate failed])
+            ])
+        ])
     ])
     # Check for libpng.
     AC_ARG_WITH([libpng],
@@ -88,8 +95,15 @@ AC_SDL_MAIN_WORKAROUND([
         [Build without libpng @<:@default=check@:>@]),
     [],
     [
+        [with_libpng=check]
+    ])
+    AS_IF([test "x$with_libpng" != xno], [
         AC_CHECK_LIB(z, zlibVersion)
-        AC_CHECK_LIB(png, png_get_io_ptr)
+        AC_CHECK_LIB(png, png_get_io_ptr, [], [
+            AS_IF([test "x$with_libpng" != xcheck], [AC_MSG_FAILURE(
+                [--with-libpng was given, but test for libpng failed])
+            ])
+        ])
     ])
     AC_CHECK_LIB(m, log)