Merge #518: Summarize build options after running configure

Merge #518: Summarize build options after running configure

3965027 Summarize build options in configure script (Evan Klitzke)

Pull request description:

  This is a trivial build system change to summarize the build options after running configure.

  Example output:
  ```
  $ ./configure
  ....
  <many lines omitted>
  ...
  config.status: src/libsecp256k1-config.h is unchanged
  config.status: executing depfiles commands
  config.status: executing libtool commands

  Build Options:
    with endomorphism   = no
    with ecmult precomp = yes
    with jni            = no
    module ecdh         = no
    module recovery     = no

    asm                 = x86_64
    bignum              = gmp
    field               = 64bit
    scalar              = 64bit

    CC                  = gcc
    CFLAGS              = -g -O2 -W -std=c89 -pedantic -Wall -Wextra -Wcast-align -Wnested-externs -Wshadow -Wstrict-prototypes -Wno-unused-function -Wno-long-long -Wno-overlength-strings -fvisibility=hidden -O3
    CPPFLAGS            =
    LDFLAGS             =
  ```

  I tried to just include the configure options that looked interesting; let me know if there are any I didn't include that I should have.

Tree-SHA512: 428381654d772f76bc81210d39ba5c3f07a94dc6a6378a02ccc6f23ebce7f501896268bcd2e94e2b0d8aea54c9c70c44a9238a0f0960600f463b1e2847c7ed1f