The MCPU_OPTS conditional block currently incorrectly sets ABI=64 for

all ARCH strings that in 64. GMP expects mode64, mode32 or 32 as valid values
for the ABI option [1]. Using an invalid value causes the following
build error on powerpc64:

    configure: error: ABI=64 is not among the following valid choices:
    mode64 mode32 32

This is the minimum change to set the correct "mode64" ABI value when
ARCH is powerpc64, while also keeping all other semantics the same. A
more complete refactor of this port is possible, but would need an
exp-run to test for regressions.

The change was tested on powerpc64 (thanks justin) and amd64 (for regression)

While I'm here:

- Remove indefinite article from COMMENT

[1] https://gmplib.org/manual/Build-Options.html

PR:		ports/179127
Submitted by:	jhibbits
Reviewed by:	jhibbits, ak
Approved by:	maintainer timeout (6 months)

math/gmp/Makefile

@@ -8,7 +8,7 @@ MASTER_SITES=	${MASTER_SITE_GNU}
 MASTER_SITE_SUBDIR=	gmp
 
 MAINTAINER=	ale@FreeBSD.org
-COMMENT=	A free library for arbitrary precision arithmetic
+COMMENT=	Free library for arbitrary precision arithmetic
 
 LICENSE=	LGPL3
 
@@ -34,7 +34,11 @@ INFO=		gmp
 .if ! ${PORT_OPTIONS:MCPU_OPTS}
 CONFIGURE_ARGS+=--build=${ARCH}-portbld-freebsd${OSREL}
 .elif ${ARCH:S/64//} != ${ARCH}
+.if ${ARCH} == powerpc64
+CONFIGURE_ENV+=	ABI="mode64"
+.else
 CONFIGURE_ENV+=	ABI="64"
+.endif
 .else
 CONFIGURE_ENV+=	ABI="32"
 .endif