doc: fix compiler warning in libcurl.m4

[Karlson2k]

Current test for curl_free() may produce warnings with strict compiler flags or even with default compiler flags with upcoming versions like this:

configure:38960: checking for curl_free
configure:38960: clang-17 -o conftest -fno-strict-aliasing -O0 -Wall -Wnull-dereference -Wdeclaration-after-statement -Wimplicit -Wnested-externs -Wredundant-decls -Wpoison-system-directories -ggdb3 -Wextra -Wdouble-promotion -Wformat=2 -Wno-format-nonliteral -Wformat-security -Wmissing-include-dirs -Wfloat-equal -Wshadow -Wpointer-arith -Wshadow-all -Wbad-function-cast -Wcast-qual -Wwrite-strings -Wconversion -Wcast-align -Waggregate-return -Wstrict-prototypes -Wold-style-definition -Wmissing-declarations -Wmissing-prototypes -Wuninitialized -Winit-self -Wshift-negative-value -Wswitch-enum -Wstrict-overflow=4 -Walloca -Warray-bounds -Wpacked -Wvariadic-macros -Wundef -Wanon-enum-enum-conversion -Warray-bounds-pointer-arithmetic -Wassign-enum -Wbit-int-extension -Wbitfield-enum-conversion -Wparentheses -Wbool-operation -Wcast-function-type -Wcomma -Wcomment -Wcompound-token-split -Wconditional-uninitialized -Wdeprecated -Wdocumentation-pedantic -Wempty-init-stmt -Wenum-conversion -Wexpansion-to-defined -Wflexible-array-extensions -Wloop-analysis -Wformat-pedantic -Wformat-type-confusion -Wfour-char-constants -Wgcc-compat -Wgnu-anonymous-struct -Wgnu-compound-literal-initializer -Wgnu-conditional-omitted-operand -Wgnu-designator -Wgnu-empty-initializer -Wgnu-empty-struct -Wgnu-flexible-array-initializer -Wgnu-folding-constant -Wgnu-null-pointer-arithmetic -Wgnu-pointer-arith -Wgnu-redeclared-enum -Wgnu-union-cast -Wgnu-variable-sized-type-not-at-end -Widiomatic-parentheses -Wmissing-noreturn -Wmissing-variable-declarations -Wnested-anon-types -Wnewline-eof -Wover-aligned -Wredundant-parens -Wshift-sign-overflow -Wtautological-compare -Wunaligned-access -Wunused -Wzero-as-null-pointer-constant -Wzero-length-array -Wused-but-marked-unused   -D_GNU_SOURCE -D_XOPEN_SOURCE=700   -Wl,--warn-common  conftest.c  -lcurl >&5
conftest.c:127:16: warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
char curl_free ();
               ^
                void
1 warning generated.
configure:38960: $? = 0
configure:38960: result: yes

These warning could turned into errors by -Werror or similar flags.
Such warnings/errors are avoided by this patch.

With this patch the check is based on header, not libcurl binary, but having different versions of libcurl binary and libcurl header could easily lead to many various problems, so quick failure on build-time (if curl_free() is present in header, but absent in the library binary) could be a better option then hard-to-debug failures on run-time.

[bagder]

Thanks!