Allow overriding default flags

Summary:
```
Right now, it's not easy to reduce the optimization level with CFLAGS
because configure overwrites any optimization flag with -O3. The
automake documentation states that:

    The reason ‘$(CPPFLAGS)’ appears after ‘$(AM_CPPFLAGS)’ or
‘$(mumble_CPPFLAGS)’ in the compile command is that users should always
have the last say.

and also that it's incorrect to redefine CFLAGS in the first place

    You should never redefine a user variable such as CPPFLAGS in
Makefile.am. [...] You should not add options to these user variables
within configure either, for the same reason

With this PR CFLAGS is still redefined, but user-provided flags appear
after the default CFLAGS which means that they override the default
flags (at least in clang and gcc). Otherwise, the default configuration
is not changed. This also means that if CFLAGS are defined by the user,
then -g is not added (which does not seem to make much sense). In order
to keep the -O3 despite the reordering we need to explicitly tell
autoconf to not append -O2 by setting the default to -g with :
${CFLAGS="-g"} as per the manual
```

Backport of secp256k1 [[bitcoin-core/secp256k1#700 | PR700]].

Test Plan:
  make check
  ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6309

src/secp256k1/CMakeLists.txt

@@ -13,8 +13,6 @@ if(NOT CMAKE_BUILD_TYPE)
 	set(__NO_USER_CMAKE_BUILD_TYPE ON CACHE BOOL "True if the user didn't set a build type on the command line")
 endif()
 
-set(CMAKE_C_FLAGS_RELWITHDEBINFO "-g -O3")
-
 option(SECP256K1_ENABLE_COVERAGE "Enable coverage" OFF)
 option(SECP256K1_ENABLE_BRANCH_COVERAGE "Enable branch coverage" OFF)
 

src/secp256k1/configure.ac

@@ -7,6 +7,11 @@ AH_TOP([#ifndef LIBSECP256K1_CONFIG_H])
 AH_TOP([#define LIBSECP256K1_CONFIG_H])
 AH_BOTTOM([#endif /*LIBSECP256K1_CONFIG_H*/])
 AM_INIT_AUTOMAKE([foreign subdir-objects])
+
+# Set -g if CFLAGS are not already set, which matches the default autoconf
+# behavior (see PROG_CC in the Autoconf manual) with the exception that we don't
+# set -O2 here because we set it in any case (see further down).
+: ${CFLAGS="-g"}
 LT_INIT
 
 dnl make the compilation flags quiet unless V=1 is used
@@ -19,10 +24,6 @@ AC_PATH_TOOL(RANLIB, ranlib)
 AC_PATH_TOOL(STRIP, strip)
 AX_PROG_CC_FOR_BUILD
 
-if test "x$CFLAGS" = "x"; then
-  CFLAGS="-g"
-fi
-
 AM_PROG_CC_C_O
 
 AC_PROG_CC_C89
@@ -63,11 +64,11 @@ case $host_os in
    ;;
 esac
 
-CFLAGS="$CFLAGS -W"
+CFLAGS="-W $CFLAGS"
 
 warn_CFLAGS="-std=c89 -pedantic -Wall -Wextra -Wcast-align -Wnested-externs -Wshadow -Wstrict-prototypes -Wno-unused-function -Wno-long-long -Wno-overlength-strings"
 saved_CFLAGS="$CFLAGS"
-CFLAGS="$CFLAGS $warn_CFLAGS"
+CFLAGS="$warn_CFLAGS $CFLAGS"
 AC_MSG_CHECKING([if ${CC} supports ${warn_CFLAGS}])
 AC_COMPILE_IFELSE([AC_LANG_SOURCE([[char foo;]])],
     [ AC_MSG_RESULT([yes]) ],
@@ -76,7 +77,7 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[char foo;]])],
     ])
 
 saved_CFLAGS="$CFLAGS"
-CFLAGS="$CFLAGS -fvisibility=hidden"
+CFLAGS="-fvisibility=hidden $CFLAGS"
 AC_MSG_CHECKING([if ${CC} supports -fvisibility=hidden])
 AC_COMPILE_IFELSE([AC_LANG_SOURCE([[char foo;]])],
     [ AC_MSG_RESULT([yes]) ],
@@ -190,10 +191,10 @@ AM_CONDITIONAL([VALGRIND_ENABLED],[test "$enable_valgrind" = "yes"])
 
 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="$LDFLAGS --coverage"
+    CFLAGS="-O0 --coverage $CFLAGS"
+    LDFLAGS="--coverage $LDFLAGS"
 else
-    CFLAGS="$CFLAGS -O3"
+    CFLAGS="-O2 $CFLAGS"
 fi
 
 if test x"$use_ecmult_static_precomputation" != x"no"; then
@@ -211,7 +212,7 @@ if test x"$use_ecmult_static_precomputation" != x"no"; then
 
   warn_CFLAGS_FOR_BUILD="-Wall -Wextra -Wno-unused-function"
   saved_CFLAGS="$CFLAGS"
-  CFLAGS="$CFLAGS $warn_CFLAGS_FOR_BUILD"
+  CFLAGS="$warn_CFLAGS_FOR_BUILD $CFLAGS"
   AC_MSG_CHECKING([if native ${CC_FOR_BUILD} supports ${warn_CFLAGS_FOR_BUILD}])
   AC_COMPILE_IFELSE([AC_LANG_SOURCE([[char foo;]])],
       [ AC_MSG_RESULT([yes]) ],