Merge r3081 (add two new custom macros for finding libraries/headers)

Originally committed to SVN as r3823.

aegisub/acinclude.m4

@@ -1,4 +1,5 @@
 m4_include([m4macros/ac_agi.m4])
+m4_include([m4macros/agi_find_libheader.m4])
 m4_include([m4macros/ac_agi_mdcpucfg.m4])
 m4_include([m4macros/ac_flag.m4])
 m4_include([m4macros/acx_pthread.m4])

aegisub/m4macros/agi_find_libheader.m4

@@ -0,0 +1,58 @@
+AC_DEFUN([AGI_FIND_HEADER],[
+  file=`echo $2 | $as_tr_sh`
+
+  aegisub_save_CPPFLAGS="$CPPFLAGS"
+
+  for dir in $3; do
+    vdir=`echo $dir | $as_tr_sh`
+    CPPFLAGS="-I$dir"
+    AC_CACHE_CHECK(
+      [for $2 in $dir],
+      [agi_cv_header_${vdir}_${file}],
+      [AC_COMPILE_IFELSE(
+        [AC_LANG_PROGRAM([#include <$2>])],
+        [eval agi_cv_header_${vdir}_${file}="yes"; found="${dir}"],
+        [eval agi_cv_header_${vdir}_${file}="no"; found=""])
+    ])
+
+    if test -n "$found"; then
+      break;
+    fi
+  done
+
+  if test -n "$found"; then
+    $1_CFLAGS="-I$found"
+  fi
+  CPPFLAGS="$aegisub_save_CPPFLAGS"
+])
+
+
+AC_DEFUN([AGI_FIND_LIB],[
+  aegisub_save_LDFLAGS="$LDFLAGS"
+
+  if test -n $3; then
+    LDDIR="-L$3"
+  fi
+
+  for lib in $2; do
+    vlib=`echo $lib | $as_tr_sh`
+    LDFLAGS="$LDDIR -l$lib"
+    AC_CACHE_CHECK(
+      [for -l${lib}],
+      [agi_cv_lib_${vlib}],
+      [AC_LINK_IFELSE(
+        [AC_LANG_PROGRAM()],
+        [eval agi_cv_lib_${vlib}="yes"; found="${lib}"],
+        [eval agi_cv_lib_${vlib}="no"; found=""])
+    ])
+
+    if test -n "$found"; then
+      break;
+    fi
+  done
+
+  if test -n "$found"; then
+    $1_LDFLAGS="$LDDIR -l$found"
+  fi
+  LDFLAGS="$aegisub_save_LDFLAGS"
+])