Merge #2978: [Build] Ubuntu Bionic gmp detection workaround

3e4c231 Build: Ubuntu Bionic gmp detection workaround (Fuzzbawls)

Pull request description:

  Ubuntu Bionic's libgmp-dev package doesn't include pkgconfig files, so we need to do some fallback checking during the configure process.

  Bionic is still used for PPA and SnapCraft nightly builds, and we do still support Bionic in terms of glibc.

ACKs for top commit: 3e4c231
  Liquid369:
    ACK 3e4c231
  Liquid369:
    ACK 3e4c231
  Duddino:
    ACK 3e4c231

Tree-SHA512: 202bc3515a4b0ecf001e5b605f05e35e40fd1e9f08310c2ec0b6b37cee22a9a6d9f6a332384b9696bd8aa85406b9e34adb61d256c5cefa75b3190f969a2d40a6

Author: Fuzzbawls

configure.ac

@@ -1169,7 +1169,14 @@ if test x$use_pkgconfig = xyes; then
           PKG_CHECK_MODULES([EVENT_PTHREADS], [libevent_pthreads], [], [AC_MSG_ERROR([libevent_pthreads not found.])])
         fi
         PKG_CHECK_MODULES([SODIUM], [libsodium], [], [AC_MSG_ERROR([libsodium not found.])])
-        PKG_CHECK_MODULES([GMP], [gmp], [], [AC_MSG_ERROR([libgmp not found.])])
+        dnl Ubuntu Bionic's libgmp-dev package doesn't include pkgconfig files,
+        dnl so we need to do some gymnastics here.
+        dnl TODO: Remove this drill down once Ubuntu Bionic is no longer supported.
+        PKG_CHECK_MODULES([GMP], [gmp], [], [
+          AC_CHECK_HEADER([gmp.h], [
+            AC_CHECK_LIB([gmp], [__gmpz_init], [LIBS="$LIBS -lgmp"], [AC_MSG_ERROR([libgmp missing])])
+          ], [AC_MSG_ERROR([libgmp headers missing])])
+        ])
       fi
 
       if test "x$use_zmq" = "xyes"; then