build: Fix boost build on some platforms

When the libpath doesn't line up with the value from config.sub, we don't find
the correct path to boost's libs. This adds a hack to try another path before
giving up.

Should close #3219.

Rebased-From: 54c7df81

doc/release-notes.md

@@ -67,6 +67,7 @@ Miscellaneous:
 - key.cpp: fail with a friendlier message on missing ssl EC support
 - Remove bignum dependency for scripts
 - Upgrade OpenSSL to 1.0.1i (see https://www.openssl.org/news/secadv_20140806.txt - just to be sure, no critical issues for Bitcoin Core)
+- Fix boost detection in build system on some platforms
 
 Credits
 --------

src/m4/ax_boost_base.m4

@@ -112,6 +112,12 @@ if test "x$want_boost" = "xyes"; then
         ;;
     esac
 
+    dnl some arches may advertise a cpu type that doesn't line up with their
+    dnl prefix's cpu type. For example, uname may report armv7l while libs are
+    dnl installed to /usr/lib/arm-linux-gnueabihf. Try getting the compiler's
+    dnl value for an extra chance of finding the correct path.
+    libsubdirs="lib/`$CXX -dumpmachine 2>/dev/null` $libsubdirs"
+
     dnl first we check the system location for boost libraries
     dnl this location ist chosen if boost libraries are installed with the --layout=system option
     dnl or if you install boost with RPM