add FIND_LIBWEBP

some platforms are missing a .pc file for libwebp ... add a FIND_LIBWEBP
macro as a fallback

see https://github.com/jcupitt/libvips/issues/440

acinclude.m4

@@ -684,3 +684,123 @@ fi
 
 ])dnl
 
+
+dnl From FIND_MOTIF and ACX_PTHREAD, without much understanding
+dnl
+dnl FIND_LIBWEBP[ACTION-IF-FOUND[, ACTION-IF-NOT-FOUND]]
+dnl ---------------------------------------------------
+dnl
+dnl Find webp libraries and headers ... useful for platforms which are missing
+dnl the webp .pc file
+dnl
+dnl Put compile stuff in LIBWEBP_INCLUDES
+dnl Put link stuff in LIBWEBP_LIBS
+dnl Define HAVE_LIBWEBP if found.
+dnl
+AC_DEFUN([FIND_LIBWEBP], [
+AC_REQUIRE([AC_PATH_XTRA])
+
+LIBWEBP_INCLUDES=""
+LIBWEBP_LIBS=""
+
+AC_ARG_WITH(libwebp, 
+  AS_HELP_STRING([--without-libwebp], [build without libwebp (default: test)]))
+# Treat --without-libwebp like --without-libwebp-includes --without-libwebp-libraries.
+if test "$with_libwebp" = "no"; then
+  LIBWEBP_INCLUDES=no
+  LIBWEBP_LIBS=no
+fi
+
+AC_ARG_WITH(libwebp-includes,
+  AS_HELP_STRING([--with-libwebp-includes=DIR], [libwebp includes are in DIR]),
+  LIBWEBP_INCLUDES="-I$withval")
+AC_ARG_WITH(libwebp-libraries,
+  AS_HELP_STRING([--with-libwebp-libraries=DIR], [libwebp libraries are in DIR]),
+  LIBWEBP_LIBS="-L$withval -lwebp")
+
+AC_MSG_CHECKING(for libwebp)
+
+# Look for webp/decode.h
+if test "$LIBWEBP_INCLUDES" = ""; then
+  # Check the standard search path
+  AC_TRY_COMPILE([#include <webp/decode.h>],[int a;],[
+    LIBWEBP_INCLUDES=""
+  ], [
+    # webp/decode.h is not in the standard search path, try
+    # $prefix
+    libwebp_save_INCLUDES="$INCLUDES"
+
+    INCLUDES="-I${prefix}/include $INCLUDES"
+
+    AC_TRY_COMPILE([#include <webp/decode.h>],[int a;],[
+      LIBWEBP_INCLUDES="-I${prefix}/include"
+    ], [
+      LIBWEBP_INCLUDES="no"
+    ])
+
+    INCLUDES=$libwebp_save_INCLUDES
+  ])
+fi
+
+# Now for the libraries
+if test "$LIBWEBP_LIBS" = ""; then
+  libwebp_save_LIBS="$LIBS"
+  libwebp_save_INCLUDES="$INCLUDES"
+
+  LIBS="-lwebp $LIBS"
+  INCLUDES="$LIBWEBP_INCLUDES $INCLUDES"
+
+  # Try the standard search path first
+  AC_TRY_LINK([#include <webp/decode.h>],[WebPInitDecoderConfig(0)], [
+    LIBWEBP_LIBS="-lwebp"
+  ], [
+    # libwebp is not in the standard search path, try $prefix
+
+    LIBS="-L${prefix}/lib $LIBS"
+
+    AC_TRY_LINK([#include <webp/decode.h>],[WebPInitDecoderConfig(0)], [
+      LIBWEBP_LIBS="-L${prefix}/lib -lwebp"
+    ], [
+      LIBWEBP_LIBS=no
+    ])
+  ])
+
+  LIBS="$libwebp_save_LIBS"
+  INCLUDES="$libwebp_save_INCLUDES"
+fi
+
+AC_SUBST(LIBWEBP_LIBS)
+AC_SUBST(LIBWEBP_INCLUDES)
+
+# Print a helpful message
+libwebp_libraries_result="$LIBWEBP_LIBS"
+libwebp_includes_result="$LIBWEBP_INCLUDES"
+
+if test x"$libwebp_libraries_result" = x""; then
+  libwebp_libraries_result="in default path"
+fi
+if test x"$libwebp_includes_result" = x""; then
+  libwebp_includes_result="in default path"
+fi
+
+if test "$libwebp_libraries_result" = "no"; then
+  libwebp_libraries_result="(none)"
+fi
+if test "$libwebp_includes_result" = "no"; then
+  libwebp_includes_result="(none)"
+fi
+
+AC_MSG_RESULT([libraries $libwebp_libraries_result, headers $libwebp_includes_result])
+
+# Finally, execute ACTION-IF-FOUND/ACTION-IF-NOT-FOUND:
+if test "$LIBWEBP_INCLUDES" != "no" && test "$LIBWEBP_LIBS" != "no"; then
+  AC_DEFINE(HAVE_LIBWEBP,1,[Define if you have libwebp libraries and header files.])
+  $1
+else
+  LIBWEBP_INCLUDES=""
+  LIBWEBP_LIBS=""
+  $2
+fi
+
+])dnl
+

configure.ac

@@ -673,6 +673,8 @@ if test x"$with_cfitsio" != "xno"; then
 fi
 
 # libwebp
+# some platforms, like ubuntu 12.04, are missing the .pc files for libwebp, so
+# we fall back to FIND_LIBWEBP
 AC_ARG_WITH([libwebp], 
   AS_HELP_STRING([--without-libwebp], [build without libwebp (default: test)]))
 
@@ -681,9 +683,14 @@ if test x"$with_libwebp" != "xno"; then
     [AC_DEFINE(HAVE_LIBWEBP,1,[define if you have libwebp installed.])
      with_libwebp=yes
      PACKAGES_USED="$PACKAGES_USED libwebp"],
-    [AC_MSG_WARN([libwebp not found; disabling libwebp support])
+    [FIND_LIBWEBP([
+       with_libwebp="yes (found by search)"
+     ], [
+       AC_MSG_WARN([libwebp not found; disabling WEBP support])
        with_libwebp=no
      ])
+    ]
+  )
 fi
 
 # pangoft2