build: Fix OSX build when using Homebrew and qt5

Qt5 is bottled, so configure won't find it without some help. Use
brew to find out its prefix.

Also, qt5 added the host_bins variable to pkg-config, use it.

build-aux/m4/bitcoin_qt.m4

@@ -152,6 +152,13 @@ AC_DEFUN([BITCOIN_QT_CONFIGURE],[
   fi
   CPPFLAGS=$TEMP_CPPFLAGS
   ])
+
+  if test x$use_pkgconfig$qt_bin_path = xyes; then
+    if test x$bitcoin_qt_got_major_vers = x5; then
+      qt_bin_path="`$PKG_CONFIG --variable=host_bins Qt5Core 2>/dev/null`"
+    fi
+  fi
+
   BITCOIN_QT_PATH_PROGS([MOC], [moc-qt${bitcoin_qt_got_major_vers} moc${bitcoin_qt_got_major_vers} moc], $qt_bin_path)
   BITCOIN_QT_PATH_PROGS([UIC], [uic-qt${bitcoin_qt_got_major_vers} uic${bitcoin_qt_got_major_vers} uic], $qt_bin_path)
   BITCOIN_QT_PATH_PROGS([RCC], [rcc-qt${bitcoin_qt_got_major_vers} rcc${bitcoin_qt_got_major_vers} rcc], $qt_bin_path)

configure.ac

@@ -240,13 +240,26 @@ case $host in
 
        AC_CHECK_PROG([BREW],brew, brew)
        if test x$BREW = xbrew; then
-         dnl add default homebrew paths
-         openssl_prefix=`$BREW --prefix openssl`
-         bdb_prefix=`$BREW --prefix berkeley-db4`
+         dnl These Homebrew packages may be bottled, meaning that they won't be found
+         dnl in expected paths because they may conflict with system files. Ask
+         dnl Homebrew where each one is located, then adjust paths accordingly.
+         dnl It's safe to add these paths even if the functionality is disabled by
+         dnl the user (--without-wallet or --without-gui for example).
+
+         openssl_prefix=`$BREW --prefix openssl 2>/dev/null`
+         bdb_prefix=`$BREW --prefix berkeley-db4 2>/dev/null`
+         qt5_prefix=`$BREW --prefix qt5 2>/dev/null`
+         if test x$openssl_prefix != x; then
            export PKG_CONFIG_PATH="$openssl_prefix/lib/pkgconfig:$PKG_CONFIG_PATH"
+         fi
+         if test x$bdb_prefix != x; then
            CPPFLAGS="$CPPFLAGS -I$bdb_prefix/include"
            LIBS="$LIBS -L$bdb_prefix/lib"
          fi
+         if test x$qt5_prefix != x; then
+           export PKG_CONFIG_PATH="$qt5_prefix/lib/pkgconfig:$PKG_CONFIG_PATH"
+         fi
+       fi
      else
        case $build_os in
          *darwin*)