Merge bitcoin-core#561: Respect LDFLAGS and #undef STATIC_PRECOMPUTATION if using basic config

dbed75d Undefine `STATIC_PRECOMPUTATION` if using the basic config (DesWurstes)
310111e Keep LDFLAGS if `--coverage` (DesWurstes)

Pull request description:

  Update: **This is a trimmed pull request with strong rationale.**

  - Adding `--coverage` shouldn't reset `LDFLAGS`, this is definitely a typo
  - The basic configuration should undefine `STATIC_PRECOMPUTATION`, as generating it is not supported and it complicates bitcoin-core#549

Tree-SHA512: 29f0dd4c870ec60d535346446b453da459ca843ed1265c2bc966bf0fcbdf3c5c79f9e48a419662e81d790a7003f8877a16e2a5a74aa5c0b79645e15ad56a0f66

Author: gmaxwell

configure.ac

@@ -156,7 +156,7 @@ AC_CHECK_TYPES([__int128])
 if test x"$enable_coverage" = x"yes"; then
     AC_DEFINE(COVERAGE, 1, [Define this symbol to compile out all VERIFY code])
     CFLAGS="$CFLAGS -O0 --coverage"
-    LDFLAGS="--coverage"
+    LDFLAGS="$LDFLAGS --coverage"
 else
     CFLAGS="$CFLAGS -O3"
 fi

src/basic-config.h

@@ -10,6 +10,7 @@
 #ifdef USE_BASIC_CONFIG
 
 #undef USE_ASM_X86_64
+#undef USE_ECMULT_STATIC_PRECOMPUTATION
 #undef USE_ENDOMORPHISM
 #undef USE_FIELD_10X26
 #undef USE_FIELD_5X52